Dimpled and darkly handsome Mark Dalton arrived in Pine Valley in 1976. He immediately caught the eye of vixen Erica Kane. When Erica introduced Mark to her mother, Mona, Mona liked him, but she also had an odd sense of familiarity. After a while, she realized just why he seemed familiar. She managed to track down Erica and Mark moments before they planned to have sex and told them the awful truth -- they were brother and sister!! Erica's father Eric had had an affair with his secretary. She had then married a wealthy man named Dalton and Mark was raised as his son. Bewildered, Erica and Mark parted ways but eventually became friends.
Mark began dating Ellen Shepherd. There was a great deal of anxiety on her part because she was over 10 years his senior and had an adult daughter. They fell in love, but when he proposed, she was wary about entering married life again. While they were apart, Mark met Brooke English while he was teaching a music class at PVU. They began a relationship, even living together at one point, but they split when Ellen agreed to marry him.
Problems began to arise over Mark's desire to have a child. Ellen was against raising another baby at her age. Mark spent more and more time at his work, and drifted into an affair with college student Pamela Kingsley. She thought they were in love, but his heart was still with Ellen. When he broke the news to Pamela, she took an overdose of pills and nearly died. Ellen discovered their affair and angrily asked Mark for a divorce.
Mark, under pressure as a songwriter, and missing Ellen, began to take more and more cocaine to ease his burdens. Soon he had spiraled into full-blown addiction. While he was on the run from the police, Erica hid him out in her attic, but she soon noticed what horrible shape he was in. With help from Ellen, Brooke, and medical professionals, Erica held an intervention which forced Mark to accept his dependence on drugs. He went to rehab and after serious effort, vanquished his demons.
Ellen was by this time a veteran of another failed marriage (to Ross Chandler), which made her realize that she'd never stopped loving Mark. She and Mark reconciled, and after a quick remarriage and a warm goodbye to family and friends, they moved to Japan.
Mark visited in early 1995 for Mona's funeral, and although he was not present when Erica suffered through painkiller addiction in 1995 and early 1996, he did return when she had an alcohol problem in 2004. With the help of his nieces (her daughters) Bianca and Kendall, and Erica's friends, he held an intervention similar to what she had done for him nearly 20 years earlier. Erica went to rehab and Mark returned to Hong Kong, where he played piano on a cruise ship.
Mark returned in early 2005 for the show's 35th anniversary, and to surprise Bianca after she was reunited with her daughter Miranda.