Lord Marcus Beaton is an English Lord, described as a handsomer British version of Nate Archibald, who Blair falls for after meeting him at The Yale Club. After a whirlwind romance, Blair sees herself marrying him and buys an expensive wedding dress. However, rumors begin to circulate on Gossip Girl that he is engaged to someone else. When Blair visits him in England, she discovers that he is more interested in his cousin, Camilla. After Blair tries unsuccessfully to keep Marcus' attention, she leaves England alone.
In the premiere Summer Kind of Wonderful, Blair returns to the US with Marcus on her arm. However, he has introduced himself as an American college student at Georgetown named James. She later admits to Serena that she doesn't like him at all and is only using him to make Chuck jealous. After talking with James, Chuck learns he is allegedly a student at Princetown. But after bringing this up to Blair, she insists that he goes to Georgetown. Suspicious at who James really is, Chuck enlists Eric and his PI to dig up any dirt about him, and they discover that he isn't a student at any big school. Later at The White Party, Marcus realizes that Blair is using him to make Chuck jealous and confronts her about it. She admits it and also adds that she finds him boring. She later apologizes to him and he reveals that his true identity is the British Lord Marcus Beaton, and he apologizes for lying about who he is.
When Blair returns to New York with Marcus, she becomes disheartened when he doesn't want her to meet his family. As a solution, she throws a last minute party that he can bring his parents to. Meanwhile, Chuck meets Marcus' stepmother, Duchess Catherine Beaton and brings her as his plus one to Blair's party, knowing Blair would rip into her. His plan works and Blair insults Catherine several times before realizing who she is. She tries to apologize but Catherine informs her that she will never end up with Marcus because she is a lowly commoner. However, Blair is able to blackmail Catherine into liking her after she catches her having sex with Nate Archibald (Never Been Marcused). In The Dark Night, Blair becomes increasingly disappointed with Marcus' lack of interest in sex. During a party in her penthouse, the city experiences a blackout and she makes out with Chuck; which Marcus walks in on when the lights come on. Blair denies knowing it was Chuck at first, but later admits she knew it was him. She tells Marcus to stop treating her delicately and actually act like he wants her. Soon after, Vanessa Abrams catches Marcus about to hook up with Catherine and she photographs the two together. She shows Blair the evidence, and Blair uses it to blackmail the Beatons to not out where Nate's father, Howard Archibald, is hiding from the feds. She also breaks up with Marcus in the process (The Ex-Files).
In Chuck In Real Life, Vanessa threatens to expose Catherine and Marcus' affair to blackmail Blair, but Blair is able to delete the evidence before she can.
- He drinks gin martinis.
- He speaks six languages.