Mark Everett Sloan (1968-2012) was originally known for being the man-whore who wrecked Addison and Derek's marriage. However, he moved to Seattle and finally managed to win Derek back as a friend and started making other friends. He even decided to become a better person and change his ways. He and Lexie Grey had an on and off relationship; even when they were not dating, he was very much in love with her. However, Mark and Lexie, along with some of the other doctors, were in a plane crash. Lexie died not long after the crash of her injuries. Mark was rescued from the woods, but spent two months trying to recover before ultimately falling into a coma. After 30 days in the coma, his life support was removed and he died.

His friendship with Derek lasted throughout childhood and into their adult years and Mark was Derek's best man at his wedding to Addison Forbes Montgomery. He says that Derek is his better half, but not better looking half. As Derek and Addison's marriage became strained, Mark and Addison grew closer, and eventually Mark slept with Addison, only for Derek to catch them in the act. Derek subsequently left New York and later Addison followed suit, leaving Mark behind.

He moved to Seattle to try to win them both back and in the end he became good friends with both.Mark first appeared in Seattle shortly after the bomb scare at the hospital with the sole intention of bringing Addison back to New York. During this visit, Meredith, Cristina, and Izzie discussed a nickname for Mark. They came up with such names as "McSexy" and "McYummy". Meredith was the one who came up with the name "McSteamy". From that moment on, the interns regularly used the nickname behind Sloan's back. He eventually found out about the nickname when Meredith, who was under the influence of painkillers, called to him "McSteamy! Woo hoo!" to which he replied, "Is that what you're calling me now? McSteamy?" She responded by saying, "Yeah, but I don't think you're supposed to know that."

Addison rejected Mark's offer to bring her back to New York and Derek rejected his apology, leaving Mark to return empty-handed to New York.

Upon his return, he was not warmly welcomed by Addison or Derek, although he desperately wanted to repair his relationships with both. He carried on a brief fling with Callie Torres before she reconciled with George O'Malley, and developed a friendship with Meredith, although he hit on her before she and Derek resumed their relationship. Mark disliked Seattle, annoyed by the weather and the perceived inadequacies of the city in comparison to Manhattan, and was also discouraged by the continued strife among himself, Derek, and Addison.

Mark's focus remains on repairing his relationships with Addison and Derek. In an effort to stray from the sexual nature of their past attempts, Mark entered a sixty-day abstinence pact with Addison with the agreement that if at the end of two months' time they had remained celibate, Addison would agree to give a relationship with Mark another chance. However thirty-two days into the pact, Mark witnessed Addison and Alex leaving an on-call room together, and realized what had happened between them. Hurt, he lied to Addison about breaking the pact. Mark and Derek remained estranged, but their relationship greatly mended, after Mark showed support and strength, in Some Kind of Miracle, while Meredith was fighting for her life. After Addison left for Los Angeles, Mark and Derek have mended their relationship, and although the subject of Mark's infidelity is occasionally brought up, they have shown mutual respect for each other, and their respective surgical skills.