Davis and Diamantopoulos will return as Joan and Rodney -- the roles they played in "The Starter Wife" miniseries last year -- in the new 10-episode series.
Davis won an Emmy Award for Outstanding Supporting Actress in a Miniseries or Movie for her portrayal of Joan in 2007.
This is the first time both Davis and Diamantopoulos have been cast as regulars in a series.
The miniseries' leading lady Debra Messing has also signed on for the show and is also now serving as executive producer.
The USA Network said the series will "focus on the post-divorce adventures of the lead character, Molly Kagan (Messing) as she begins a new chapter in her life."
Davis and Diamantopoulos play Molly's close friends, an alcoholic socialite and a gay interior decorator respectively.