In between his 1991 series stints, Slattery began his long and fruitful association with playwright Richard Greenberg, co-starring in the author's play "The Extra Man". (He subsequently played Charles Van Doren in Greenberg's examination of the 1950s quiz show scandals "Night and Her Stars" in 1995 and won critical praise in the dual roles of a father and son in "Three Days of Rain" in 1997). In 1993, Slattery made his Broadway debut as a TV show writer patterned on Larry Gelbart in Neil Simon's semi-autobiographical "Laughter on the 23rd Floor", which starred Nathan Lane as a Sid Caesar-like star. He went on to offer stalwart support to Sally Field in her return to TV drama in the miniseries "A Woman of Independent Means" (NBC, 1995). After appearing as a suitor to Mary Stuart Masterson's "Lily Dale" in the 1996 Showtime adaptation of Horton Foote's play, the actor made his feature film debut in a supporting role in "City Hall" (also 1996). Slattery portrayed Walter Mondale in the acclaimed 1998 HBO miniseries "From the Earth to the Moon". He delivered a terrific turn as Miguel Ferrer's despondent partner in a private detective agency in "Where's Marlowe?" (1998) and then segued back to the small screen as a sexy veterinarian who catches the eye of Ann Cusack's "Maggie" in that 1998 Lifetime sitcom.

After the demise of "Maggie", Slattery was in demand as a guest player on TV, turning in several memorable performances, including playing Will's estranged brother on "Will & Grace" in 1999 and a blue-collar worker romancing a movie actress in "To Catch a Falling Star" (CBS, 2000). He had a recurring role as Amy's estranged husband on the CBS drama "Judging Amy" during its first season (1999-2000) and offered an hilarious turn as a politician with kinky sex habits dating Sarah Jessica Parker's Carrie in the HBO sitcom "Sex and the City" in 2000. Slattery returned to the Great White Way in the fall of 2000 to co-star with Juliette Binoche and Liev Schreiber in a revival of Harold Pinter's "Betrayal".