Diamantopoulos began his career working in television commercials and professional theatre productions at the age of nine. At the age of eighteen, he left home to work in a series of American national tours. A short time later, he worked on
Broadway, where he played leading parts in
The Full Monty (2002) and
Les Misérables (2003). Diamantopoulos attended
East York Collegiate Institute. He played
Robin Williams in
Behind the Camera: The Unauthorized Story of Mork & Mindy alongside
Erinn Hayes. He has guest-starred in several series, including
Kevin Hill,
Charmed,
The Sopranos,
The Office,
Nip/Tuck and
Boston Legal. He played the
interior decorator in the miniseries
The Starter Wife. He had a recurring role on
State of Mind and guest-voiced on
American Dad! in 2009. On the eighth season of
24, he played U.S. president
Allison Taylor's
Chief of Staff Rob Weiss. He played
Frank Sinatra in
The Kennedys. Diamantopoulos played
Moe Howard in the 2012 film
The Three Stooges. In 2013, Diamantopoulos voiced
Mickey Mouse in
the television series with the same name, and its follow-up series
The Wonderful World of Mickey Mouse. Although
Bret Iwan has voiced Mickey in most media following
Wayne Allwine's death from
diabetes in May 2009, Diamantopoulos voiced the character in retro-styled productions, giving Mickey Mouse a voice similar to that of
Walt Disney. Most recently, he reprised the character in the 2023 short film
Once Upon a Studio, made to celebrate the centennial of
The Walt Disney Company and
Walt Disney Animation Studios. He had recurring roles on the
NBC sitcoms
Up All Night, as
Maya Rudolph's on and off lover, Julian;
The Office, as Brian, a former member of the show's documentary film crew (alongside
Zach Woods, who he would later work again with on
Silicon Valley); and on
Arrested Development, as Marky, Lindsay's face-blind environmentalist boyfriend. He appeared in the television sitcom
Episodes playing the part of TV network boss Castor Sotto, and Mr. Chris in the television adaptation of
About a Boy. In 2017, Diamantopoulos starred in the musical
Waitress on Broadway. On October 11, 2018, it was revealed by
TVLine that he would have a role in the spin-off pilot for the situation comedy
The Middle playing
Sue Heck's mercurial, charming and rich boss Nick. He has an upcoming starring role alongside
Tom Blyth in the comedy
Discussion Materials adapted from
Bill Keenan's memoir of the same name. ==Personal life==