The youngest child of Stephen and Beth Logan, teenaged Katie felt as if she were in the shadow of older sisters Brooke and Donna, which, along with a bad case of acne, made her feel insecure. Donna talked her friend, Rocco Carner, into asking Katie out to boost her morale. Katie considered Rocco her boyfriend and couldn't see that Rocco was more interested in Donna. Katie was furious when she caught Donna and Rocco kissing. Katie forgave them, but Rocco wanted to marry Donna, and put an engagement ring in Katie's coat pocket, thinking the coat was Donna's. Katie found the ring and assumed it was for her. Rocco struggled with how to let Katie down easy. Once he did, Katie was hurt, but soon able to move on with Kurt, a college classmate.
Katie and Donna were thrilled when they received an "anonymous" tip that Stephen, who had walked out on the Logans years before, was in Arizona. (It was really a hint dropped by Stephanie Forrester, who wanted Stephen to lure Beth away from Stephanie's husband, Eric.) Stephen and Beth indeed reconciled, then moved to Paris. Afterwards, Katie faded into the background for years, but could occasionally be seen working at the Insomnia Café, or when Brooke needed a babysitter.
In 2007, Katie arrived for Donna's wedding to Thorne Forrester. But Katie overheard Donna admitting that she was only marrying Thorne as a way of getting back at Stephanie for hurting Brooke. Donna ignored Katie's pleas to be honest with Thorne, so Katie went to Thorne herself, and the wedding was called off. Katie had become skilled in public relations while she was away, so when Forrester Creations offered her a PR position, Katie decided to stay in Los Angeles.
Katie's brother, Storm, shot Stephanie and framed Stephen for the crime. When it was discovered that Storm pulled the trigger, Katie convinced Stephanie not to press charges. But Storm was on a downward spiral, and took a gun to the home of Ashley Abbott, who had broken up with him. Katie followed him there, and Storm's gun went off in the struggle. Katie was critically wounded and not expected to live. A remorseful Storm, who knew Katie needed a heart transplant, took his own life in order to donate his heart to her. Katie was devastated, although grateful for her brother's ultimate sacrifice.
When Katie's body rejected Storm's heart, Katie's doctor niece, Bridget Forrester, researched ways to help Katie accept the transplant. But the prognosis was grim, so Bridget convinced her family, and fiancé Nick Marone, to make Katie's last days special. This included giving Katie the high school prom she never had, and sending her to Catalina Island, where Katie fell for Nick. Bridget came through with a therapy for Katie, who recovered. Nick and Bridget got married, and Katie hid her feelings, even officiating at their wedding. A few weeks later, Katie discovered she was pregnant. It was revealed that Nick had honored Katie's dying wish on Catalina and made love to her. Bridget left Nick and Katie to have their baby together, though Nick's mother, Jackie Marone, disapproved. Soon after, Katie miscarried. She briefly dealt with it by fixating on Nick's son, Jack, but bounced back when Nick proposed.
On Katie's wedding day, Jackie and Bridget blasted Katie for her betrayal. Katie regretted destroying Bridget's marriage, and postponed the wedding. Katie noticed that Nick was still drawn to Bridget, who had left medicine and was designing with Nick. Katie moved out and got drunk at the Café Russe, where she met Bill Spencer, Jr., a womanizer who wanted to take over the financially struggling Forrester Creations. Bill chose not to take advantage of Katie in her impaired state. Katie added modeling to her résumé at Forrester, and started seeing Bill, who told her she got past his armor like no woman had. Katie questioned Bill's reasons for seeing her when she found a DVD left by his late father, Bill Spencer, Sr., which instructed Bill to get back at Eric, Stephanie, and Donna. Bill declared his love and recognized that Katie wanted to do something important, given her second chance at life. He finally acquired Forrester Creations, made Katie its CEO, and proposed to her. Katie joyfully married Bill in her old San Fernando Valley neighborhood, with Stephen and Beth at her side.
At Forrester, Katie put Donna and Brooke in leadership positions when the Forrester men refused them, and struggled to keep the peace between the Logans and the Forresters, who tried to sabotage the company with an ugly fashion line. Meanwhile, Bill's wandering eye turned to young Steffy Forrester, who had kissed Bill before his wedding. Katie never knew that the reason Bill put the company back in Forrester hands was because Steffy had gotten a hold of Katie's engagement ring in Bill's bedroom and used it to blackmail him. Later, when Bill discovered that Liam Cooper was his son, Katie encouraged a father-son relationship between the two.
Katie discovered another side to Bill when he plotted to kill Amber Moore, who claimed Liam was her baby's father. Detesting violence because of what happened with Storm, Katie walked out and refused to return until Bill agreed to therapy. Bill reluctantly sat through sessions with Katie and Dr. Taylor Hayes. Later, it appeared that Katie and Bill had worked their differences out off-screen. When Steffy's brother, Thomas Forrester, was missing at sea, a desperate Steffy turned to Bill for help. After Thomas was found, Bill and Steffy declared their love. Katie's surprise vow renewal ceremony inadvertently thwarted Bill's plan to tell Katie he was leaving. Taylor, appalled that her daughter Steffy wanted to be with Bill, told Katie about the affair. Katie had a heart attack while confronting Bill. In the hospital, Katie demanded that Bill choose between her and Steffy. Bill chose Katie, but became more involved in helping Steffy, who had fallen in love with Liam.
Katie was enraged when Bill gave Steffy a seat on the Spencer board, and, refused to travel with Steffy when Bill arranged a "family vacation" to Cabo San Lucas, where Steffy was injured in an ATV accident. Katie, knowing that Liam wanted to leave Steffy, was suspicious when Steffy's MRI stated she had a blood clot that would become life-threatening under stress. Katie's investigation revealed that Bill had faked Steffy's MRI. Bill confirmed he had continually helped Steffy to keep Liam from his first love, Katie's niece Hope Logan. Bill locked Katie in a tower to prevent her from telling Liam, but she escaped, and turned her back on Bill after exposing him to Liam.
Again, Katie and Bill seemed to resolve their issues off-screen, resuming a happy marriage until Katie announced she was pregnant. Bill demanded she get an abortion. When Katie refused, Bill broke down and admitted he was afraid that she would die trying to have a baby given her heart condition. Katie insisted it was worth the risk. Months later, Katie discovered Hope's father, Deacon Sharpe, in Bill's office and went into premature labor upon finding out Bill had bribed a warden and sprung Deacon from jail to prevent Hope's wedding to Liam. Katie had a C-section and gave birth to baby Will, then flatlined after a heart attack. During her near-death experience, Katie saw Storm, who helped Katie return to the living.
Katie had trouble connecting to her baby since she couldn't forgive Bill for putting their family in jeopardy. Katie's sisters were told what Bill had done, but they agreed not to prosecute for the sake of Katie and her son. Not believing Taylor's claim that Katie was slipping into postpartum depression, Katie began to fear she would only end up orphaning Will, so she disappeared and made Bill and Brooke think she had taken off for Aspen. In reality, Katie rented a grungy Los Angeles apartment and sent an envelope for Bill to find in Aspen; in it was her wedding ring and a request for a divorce.
Katie decided, in her melancholy, that Brooke should take her place as Bill's wife and Will's mother, and arranged a balloon ride for Bill and Brooke in Aspen. Katie continued calling Taylor but wouldn't let Taylor know where she was. Repeatedly feeling that her heart was going to give out, Katie checked into the hospital. Katie refused to believe she wasn't dying, or that she had postpartum depression, but when Donna brought Will by for a visit, it struck a chord in Katie and brought her back from the brink.
Katie returned home to reclaim her life, oblivious to the connection that had formed between Bill and Brooke. When Taylor told Katie she had overheard Bill and Brooke discussing a kiss they had shared, Katie wouldn't accept it. Katie confronted Brooke, but when Bill and Brooke told Katie the kiss meant nothing, Katie felt it had happened because of her attempt to push them together, and forgave them.
Katie became uncomfortable with the amount of alcohol she saw Bill consuming, feeling he made bad decisions when he drank. So Katie staged an intervention which included Brooke, Taylor, Bill's half-sister Karen, and Karen's partner Danielle. Bill scoffed that he didn't have a problem with alcohol, but promised Katie he'd stop drinking. The intervention ended when Hope, who had found out Bill sabotaging her wedding in Italy, broke through the window with a golf club. Katie told Bill it was time he finally owned up to Hope about how he had manipulated her.
Bill and Katie weren't making love, so when Katie confided in Brooke, she arranged a sexy evening for the marrieds. Katie walked out of the seduction when she smelled booze on Bill's breath. Katie tried to make up for it later, but didn't notice that Bill's mind was elsewhere during their lovemaking.
Katie became increasingly focused on baby Will to the point of avoiding a frustrated Bill, who turned to Brooke for support. Katie admitted to Donna that she was probably overcompensating for her disappearance by doting on Will, but when Taylor reported that Bill was overseeing a relaunch of Brooke's sexy lingerie line, Katie stormed Bill and Brooke's boozy celebration just as they fell on the bed Forrester used as a prop. Katie blasted Brooke and insisted that Bill tell Brooke nothing was going to happen, but Bill turned it around on Katie, citing her “rules and regulations.” Katie later conceded she was being too strict with Bill and they tried to come to terms, but Katie didn't feel she could give Bill the sexual healing he wanted.
When Brooke called to tell Katie that Bill was spending the night at her house, but wouldn't say why, Katie raced over and found Bill in Brooke's bed. Katie listened as Brooke admitted she was caring for Bill after he'd had a drunk driving accident, and that they had tried to keep it from her. Katie raged that she'd had enough, taking off her wedding ring and demanding a divorce. However, Katie soon went into heart failure and lapsed into a coma. Brooke and Bill were told it was because Katie hadn't been taking her anti-rejection medication, and a suspicious Taylor wondered what would make Katie forget.
The first word Katie uttered upon coming out of her coma was "ring." Bill put it back on her finger, but Katie didn't notice the stolen glances between Bill and Brooke. Katie apologized for having ever suspected anything untoward of them and told Bill she trusted him. Katie was delighted when Bill bought her an expensive necklace and went out of his way to spend time with her. Katie found out Brooke was experiencing menopause and offered her support, later getting back to work at Spencer. Katie, by all accounts, was on Cloud Nine, but when Bill's assistant, Alison, asked Katie's advice on how to get Bill to take care of an issue at the office, Katie snarked "You could nag him 'til he does something about it," which seemed to go against her idyll.
After Steffy had a motorcycle accident and miscarried Liam's baby, Katie and Steffy argued about Liam's lingering feelings for Hope. Steffy collapsed in front of Katie, who rushed Steffy to the hospital. There, it was revealed that a botched D&C procedure after Steffy's miscarriage had rendered her unable to conceive again. Katie offered her unprecedented support and encouraged Steffy to tell Liam the truth. Instead, Steffy swore Katie to secrecy. When Katie discovered that Steffy went to Liam for a divorce, gave him her blessing to be with Hope, and moved to Paris, Katie confided in Brooke about Steffy's infertility.
Katie was concerned when Brooke came over with news for her and Bill, only to faint before she could deliver it. Brooke brushed it off as stress, so Katie threw Brooke a birthday party to cheer her up, even inviting Taylor as a way of fostering harmony in the family. But Katie's good faith gesture backfired when Taylor announced that Bill and Brooke had slept together, and that Brooke had miscarried Bill's baby! Katie refused to believe it and threw Taylor out, but when Brooke couldn't deny Taylor's claim, Katie went ballistic. Brooke and Bill tried to remind Katie that she had set the wheels in motion herself when she tried to set them up during her postpartum depression, but Katie responded by slapping Bill and disowning Brooke.
Bill tried to court Katie's favor by presenting her with a diamond tiara, but Katie balked and handed Bill divorce papers. Katie asked for the house and 1% of stock in Spencer Publications. Bill knew Katie could have asked for a lot more, so he agreed. Then Karen, who shared equal ownership of the company with Bill, walked in and announced that Bill now only owned 49% of Spencer. Karen, still owning 50%, was the majority stockholder and had appointed Katie CEO. Katie's first order of business was to fire Bill and have him removed by security guards!
Brooke confronted Katie when she found out, calling Katie's takeover thoughtless and impulsive. Katie listened as Brooke placed the blame for everything on her, then realized that Brooke's defense of Bill betrayed her true feelings for him, causing Katie to declare that Stephanie was right: Brooke was a slut from the Valley! Brooke smacked Katie and warned that her actions would only end up hurting Will, but Katie stood her ground. Deciding to test Brooke and Bill, Katie arranged to have them babysit Will, installing hidden cameras to catch them in any act. Donna disapproved, but Katie demanded silence from Donna, reminding Donna that she owed her for having kept quiet about Brooke and Bill.
Though Brooke and Bill had a few charged but otherwise innocent conversations as Katie watched, Katie was convinced the affair was over and approached Bill for a reconciliation. However, Bill discovered Katie's cameras and rejected her, saying he was glad they were divorcing. Katie was shocked when Brooke later came to Bill's defense, accusing Katie of abusing Bill!
Katie wasn't surprised when it turned out Bill had another long-lost son in new arrival Wyatt Fuller. When Katie saw how Wyatt had quickly come to idolize his newfound father, Katie burst Wyatt's bubble recounting not only Bill's affair with Brooke, but his affair with Steffy. Katie was intrigued when Wyatt changed his tune about Bill and fumed that, like himself, baby Will would also have to grow up without a father. Katie later found a kindred spirit in Wyatt's mother, Quinn Fuller, who had also suffered romantically at the hands of Bill.
Katie decided to take Will with her on a business trip to New York. Bill was furious and had Katie followed, but Katie worked with Danielle to fool the private investigator into tailing the wrong woman. Katie purposely came home unexpectedly and found that Brooke and Bill were now living together. Disgusted, Katie asked Brooke to choose between their sisterly bond and Bill. When Brooke declined to, Katie told Donna that it was time for her to move on and to put Brooke out of her mind.
Katie and Bill continued to spar, with Bill grumbling through their board meeting and Katie refusing to let Will visit Bill at Brooke's. Katie wasn't sure what to believe after Bill had an accident mountain climbing and returned swearing his devotion, but Katie took him back anyway, and they made love. After signing papers returning control of Spencer to Bill and guaranteeing him joint custody of Will, Katie smelled a rat and found Bill at Brooke's bragging how he had lulled Katie into signing. Katie was surprised when Brooke threw Bill out, then revealed that she had replaced the papers with a pile of leaves to protect Katie. With Katie still CEO of Spencer thanks to Brooke, she cautiously entertained forgiving her sister if she promised to stay away from Bill.