Thomas Forrester was born to Dr. Taylor Hamilton in January 1998 in the back of Thorne Forrester's car. Although he was Ridge Forrester's son, Taylor had allowed everyone to think he was Thorne's because Brooke and Thorne were manipulating both Ridge and Taylor to keep them apart, and they had convinced both Ridge and Taylor that the other did not want them. Taylor had wanted to tell the truth many times, but once Brooke announced her own pregnancy, Taylor had agreed to keep quiet. That lasted until the day of Brooke and Ridge's wedding, when Taylor finally realized that Brooke had been lying about her pregnancy, but when Brooke moved the wedding up, Taylor missed it completely, since she was giving birth to Thomas at that moment. Once Ridge learned that Thomas was his and that Brooke had lied about being pregnant and having a miscarriage to keep Ridge from being with Thomas and Taylor, Ridge had their marriage annulled, and he and Taylor married.

A bit over a year after Thomas' birth, Taylor got pregnant again, with twins. A few months before she was due to give birth, a homeless man named George infected Taylor with tuberculosis. George died, and Taylor nearly did while giving birth to two adorable twin girls. Luckily, she survived, and the twins were named Steffy and Phoebe. A lot happened during the next years of little Thomas' life. His mother and father broke up because of a crazy woman named Morgan DeWitt, who desperately wanted a child by Thomas' father. Ridge and Taylor got married again on St. Thomas, and Thomas' little sister Steffy was believed to be killed by a shark. Luckily, she was alive, and had been kidnapped by Morgan. Steffy and Taylor were saved when Ridge drove a car into Morgan's house.

Thomas was in high school when he met Caitlin Ramírez, a new intern at Forrester. The two started out as friends, but it wasn't long before the two were going out. When Thomas discovered that Caitlin was also seeing his stepbrother Rick, he was devastated, and found comfort in long-time friend and former babysitter Amber Moore. What started as friendship soon turned serious for the two of them, something that did not thrill his father and grandmother, who were not pleased that Thomas, who was 18, was involved with the mid-twenties Amber. Ridge initially got a restraining order, but since Thomas was over 18, it wasn't long before the family had to grudgingly accept Thomas and Amber together. They didn't like it, and Ridge never stopped trying to break them up.

Thomas eventually came to realize on his own that Amber wasn't right for him, and his feelings just intensified when he found out she nearly killed Ridge and Bridget in a trap set to win him back. He began to see Caitlin, but it never went very far. He was intrigued when he met Helen's daughter, Gabriela Moreno, and it wasn't long before the two were good friends, each having the shared pain of a lost mother. Thomas was overjoyed when his own mother, Taylor, came back from the dead, and he and the girls offered to share her with Gaby.

After Taylor's return, Thomas decided his father needed to pick which of his women he was going to be with. Although Thomas told Brooke he was grateful for everything she had done for him and his sisters, he asked her to please step aside so that his parents could continue their marriage. While all this was going on, Thomas also wanted to be a designer. He was hoping for a summer internship at Forrester, but Eric turned him down flat. Spectra Couture, however, didn't care that he was young and inexperienced, and hired him right away.

After advising Caitlin that he wanted to be "just friends," Thomas couldn't resist the pull Gabriela had on him, and offered to help her when she learned she was an illegal. Thomas offered to marry her, and on June 24, 2005, the two wed at the Beautiful Bells wedding chapel in Las Vegas.