Mark Robinson was introduced in Season 3 as a customer at the 50's themed Moondance Diner where Monica was employed. Overhearing a whining conversation between Rachel and Monica he offers to help Rachel get a job alongside him at Bloomingdale's (Rachel was employed at Fortunata Fashions after quitting her job at Central Perk to pursue her fashion career). Ross was jealous of Mark's relationship with Rachel and resentful of him. Mark was always considered as an obstacle to Ross and Rachel's relationship and was happy when they broke up.
After Ross and Rachel ended their relationship, it is revealed that Mark had always had a crush on Rachel and the two went on a date, much to the dismay of Gunther (he was present when Mark asked Rachel out and was so infuriated that he went and smashed some Central Perk property!). However Rachel refused to continue a romantic relationship with him due to her mourning the end of her relationship with Ross and when she realized that she was only dating Mark to get back at Ross. Since Rachel felt that being with Mark would be to get back at Ross, Mark decides to "get back at Ross with himself." At some point before Ross and Rachel broke up (In The One With Phoebe's Ex-Partner, actually.), Mark either left his job or got promoted, and was replaced by Sophie.
While Mark only appeared in season 3, his reappearance in season 10 had a major impact after he catches Rachel and Ross carrying Rachel's possessions out from Ralph Lauren (she was fired for going on an interview with Gucci). Ross doesn't remember him at first, until Rachel reminds him, in which he declares, "I hate that guy!" making him jealous again. It was then revealed that Mark is married with twins, calming Ross all of a sudden. Mark did however find Rachel a job with Louis Vuitton leading to almost moving to Paris, Ross tries to bribe Mr. Zelner into re-employing her, finally allowing her to go after realizing how nervous and scared, yet incredibly excited she was. Rachel doesn't end up going, deciding to stay with Ross at the last minute; leaving behind the job Mark acquired for her.
Ross Geller took an instant dislike to Mark upon meeting him in "The One Where Chandler Can't Remember Which Sister", taking the fact that Mark offered Rachel a job after overhearing her complaining about being unhappy with hers at the Moondance Diner as a sign that he has a crush on her. Although Rachel believes that Ross is being paranoid and over-reacting (which up to a point he admittedly was, after losing Carol to Susan), it later turns out Ross was right-Mark does have a crush on Rachel. Although she was unaware of this while she was in a relationship with Ross and only ever considered her relationship with Mark to be platonic, the paranoid Ross mistakenly suspected she was having an affair with him. In "The One Where Ross and Rachel Take A Break", Mark insists on coming to Rachel's apartment when she calls him, upset after a fight with Ross, and it is Mark's presence at the apartment which causes Ross, overhearing his voice in the background after phoning Rachel and mistakenly thinking he is there to have sex with her, to become convinced that his suspicions about her affair with Mark were right and that his relationship with her is now over, which (after he angrily hangs up) leads to him drunkenly having sex with Chloe.
The next morning, in "The One With The Morning After", Ross tries to prevent Rachel from learning about his affair with Chloe, but she finds out anyway from Gunther. This results in Ross and Rachel having a fight at her apartment and then breaking up in tears. The breakup obviously increased Ross's hatred of Mark. Mark was delighted when he found out and asked Rachel out on a date, making both Gunther and Ross angry in "The One With The Tiny T-Shirt", and Ross has to be talked out of attempting to intervene by Chandler, and is delighted when he later learns Rachel broke up with Mark after realizing she really only agreed to date him to get back at Ross for what he did (and Mark's attitude towards Ross throughout this discussion with Rachel shows that the dislike is mutual.) In "The One With Princess Consuela", Mark returned for the last time in the show where he found Ross and Rachel moving stuff out of her office. After he leaves, Ross seems to consider him a nice guy and asks Rachel if they had met. When Rachel reminds him of who Mark is, however, he gets jealous all over again and not for the first time strongly indicates his feelings for Rachel are still intact when he states that he doesn't want Rachel going to dinner with Mark, claims that maybe he'll give her a job as his 'sex-retary' and even tells Rachel he still distrusts as well as dislikes him and likely thinking back to the two times Ross and Rachel broke up and the hostility between them after the breakups that went on for a while. When Rachel comes home from dinner, she says that dinner was great and that Mark is so sweet resulting in Ross calling him a smarmy letch but the latter calms down almost instantly when Rachel announces that Mark is married with twins, and, seemingly embarrassed, wonders "Should we send something?" This seems to indicate that he has finally gotten over his jealousy and dislike of Mark, and is now considering making friends with him, but likely went back to hating him when he learns that he offered Rachel a job in Paris. Whether or not Ross, after getting back together with Rachel, ever did become friends with Mark is unknown, though unlikely, given what had happened in Season 3 between them: the breakup between Ross and Rachel and being apart from each other for 7 years, and in Season 10: giving Rachel a job in Paris, indicating that Ross will always hate Mark for being responsible for Ross and Rachel's breakup, them being apart for 7 years and everything Ross and Rachel went through for the rest of the show until the last episode.
Mark was ultimately defeated by Ross when Ross confessed his love to Rachel and she got back together with him and throwing away the job Mark got her, thus Ross finally managed to avenge his love for Rachel by busting Mark.
Mark was quite friendly with Rachel Green. They met at Monica's Moondance Diner and he helped her get a job at Bloomingdale's. They didn't stop seeing each other after Mark resigned in "The One With Phoebe's Ex-Partner". They went on a date together after Ross and Rachel broke up, but Rachel didn't take things further after kissing him when she realized she was only doing this to get back at Ross. However, when Mark offered Rachel a job in Paris, she at first accepted it and then abandoned it, deciding to stay in New York with Ross, leaving behind the job Mark acquired for her, symbolizing that Mark was never for her.
While working at Bloomingdale's, Mark had several other colleagues apart from Rachel, including his ex-boss Joanna, a lady called Nancy and Sophie. It's likely that he got on fine with Joanna, while he had a crush on Nancy and made out with her which caused Ross (who was eavesdropping and thinking Mark was trying to seduce Rachel) to break in and push Mark away and throw a love bug at him, because Ross thought Mark was acting romantic to Rachel. It appears Sophie knew Mark, despite her having taken his place as Rachel's deskmate and Joanna's assistant, and she said she loved him.
- It is obvious that Mark is not an only child because he was mentioned having a niece in Season 3 and was not married at that time.
- Ironically, Mark broke Ross and Rachel up in Season 3 and got them back together permanently in Season 10:
- Season 3: Mark broke Ross and Rachel because Ross thought Rachel was having an affair with him and later, when Ross heard him at Rachel's apartment, it led to him sleeping with Chloe and later Rachel found out and she and Ross broke up.
- Season 10: When Ross found out Mark offered Rachel a job in Paris, he tried everything in his power to make Rachel stay in New York and later at the airport, Ross confessed his love for Rachel, but rejected him and got on the plane and then when Ross returned to his apartment, he hears a message from Rachel who says she also loves him and tries to get off the plane and manages to and she appears in Ross's doorway, and there... they kiss and hug and get back together and promise to never break up again.
- The One Where Chandler Can't Remember Which Sister (First appearance)
- The One With All The Jealousy
- The One With Phoebe's Ex-Partner
- The One Where Ross And Rachel Take A Break
- The One With The Morning After (Mentioned only) (Appears in 'Previously on Friends' scene)
- The One Without The Ski Trip (Mentioned only)
- The One With The Tiny T-Shirt
- The One With The Cuffs (Mentioned only)
- The One With Rachel's New Dress (Mentioned only)
- The One With The Invitation (Mentioned in flashback)
- The One With Princess Consuela
- The One Where Estelle Dies (Mentioned only in 'Previously on Friends' part as 'he')