Baitz was born in Los Angeles, California, the son of Edward Baitz, an executive of the Carnation Company. Baitz was raised in Brazil and South Africa before the family returned to California, where he attended Beverly Hills High School. After graduation, he worked as a bookstore clerk and assistant to two producers, and the experiences became the basis for his first play, a one-acter entitled Mizlansky/Zilinsky. He again drew on his own background for his first two-act play, The Film Society, about the staff of a prep school in South Africa. Its 1987 success in L.A. led to an off-Broadway production with Nathan Lane the following year, which earned him a Drama Desk Award nomination for Outstanding New Play. This was followed by The End of the Day starring Roger Rees and The Substance of Fire with Ron Rifkin and Sarah Jessica Parker.

In 1991, Baitz wrote and directed the two-character play Three Hotels, based on his parents, for a presentation of PBS's "American Playhouse", then reworked the material for the stage, earning another Drama Desk Award nomination for Outstanding New Play for his efforts. In 1993, he co-scripted (with Howard A. Rodman) The Frightening Frammis, which was directed by Tom Cruise and aired as an episode of the Showtime anthology series Fallen Angels. Two years later, Henry Jaglom cast him as a gay playwright who achieves success at an early age - a character inspired by Baitz himself - in the film Last Summer in the Hamptons; the following year he appeared as Michelle Pfeiffer's business associate in the screen comedy One Fine Day. In 1996, he was one of the three finalists for the Pulitzer Prize for his semi-autobiographical play A Fair Country.

Subsequent stage works include Mizlansky/Zilinsky or "Schmucks", a revised version of Mizlansky/Zilinsky directed by Baitz's then-life partner Joe Mantello (1998), a new adaptation of Henrik Ibsen's Hedda Gabler (first at L.A.'s Geffen Playhouse with Annette Bening in 1999, then at Long Island's Bay Street Theater with Kate Burton in 2000, followed by a Broadway production with the same star the following year), Ten Unknowns (2001), starring Donald Sutherland and Juliana Margulies, and The Paris Letter (2005) with Ron Rifkin and John Glover. His screenplays include the adaptation of his own Substance of Fire (1996), with Tony Goldwyn and Timothy Hutton joining original cast members Rifkin and Parker, and People I Know (2003), which starred Al Pacino.

Baitz's occasional work writing for such television series as The West Wing and Alias led to his present position as creator and executive producer of the ABC TV drama Brothers & Sisters, which premiered in September 2006.

Baitz is the New School for Drama's artist in residence for the 2009-2010 school year.

Baitz is set to pen the stage adaptation of Hollywood legend Robert Evans' memoirs, The Kid Stays in the Picture and it’s sequel, The Fat Lady Sang, with award-winning Sir Richard Eyre set to direct.

Personal life

From 1990 to 2002, Baitz was the romantic partner of actor and director Joe Mantello. The Advocate wrote that the "couple's star quality lent the aura of a gay Lunt and Fontanne." Baitz is Jewish.