Mr. Treeger's first appearance is in "The One Where Heckles Dies", when the friends are cleaning Mr Heckles' apartment. He later appears in the second season holiday episode when Monica and Rachel's knob on the heater breaks off, causing their apartment to get very hot.
In season 4, "The One With The Ballroom Dancing", Joey gets Rachel into trouble with Mr Treeger and the latter threatens to throw Monica and Rachel out of their apartment for illegal subletting (a fact made clear many times throughout the series, as Monica took control of and subletted the spare room of the apartment and continued living there even after her grandmother, the real leaser, passed away). In order to avoid this, Joey agrees to help Treeger to practice his ballroom dancing to impress a lady-friend.
Mr Treeger later appears in "The One With The Free Porn", where he tells Joey and Chandler about the time he got free porn on his TV and made the mistake of turning it off. Treeger's final appearance is in "The One Where Rachel Tells..." where he calls the fire department to break down the door to Chandler and Monica's apartment. It is also revealed that he attends yoga class with Ross.
Although Treeger is mentioned, he does not appear in the final episode. In "The Last One", Monica told the gang to leave their keys for Treeger. He is the last character to be mentioned by name.