In 1973, Zien made his television debut on an episode of Love, American Style. More guest roles in television followed, and in the early 1980s, he began a stream of regular TV roles. In 1981, he appeared on Ryan's Hope as Daniel Thorne, the Producer of the fictional Proud and the Passionate soap opera on the series in which major character Kimberly Harris (Kelli Maroney) was starring on. Later that year, he began a two-year run in the freshman NBC sitcom Love, Sidney as Jason Stoller, the young, hot-shot ad agency Director who was Sidney Shorr's (Tony Randall) boss. Immediately after Love, Sidney's cancellation, Zien was cast in a similar role on the ABC sitcom Reggie, an American adaptation of the British series The Fall and Rise of Reginald Perrin. He played C.J. Wilcox, the overbearing young boss of Richard Mulligan's Reggie Potter. The series aired as a summer replacement during August and September of 1983, but was not renewed by ABC after the tryout run. Zien later starred on the short-lived CBS drama Shell Game in 1987.

Zien starred on Broadway in Ride the Winds (as Inari) (1974), The Suicide (as Victor Victorovich) (1980), Grand Hotel (as Otto Kringelein) (1989) and The Boys from Syracuse (as Dromio of Ephesus) (2002). He appeared in the City Center Encores! staged concert production of Applause as Buzz Richards in 2008.

Zien's first major role was in 1977 at the Manhattan Theater Club in Ron Cowen's, Seth Glassman's and Sandy Naishtat's musical "The Redemption Center", playing opposite Alice Playten as the young couple. Later, Chip received acclaim in the role of Marvin in william Finn's first musical, In Trousers, which ran Off-Broadway in 1979. In its sequels, March of the Falsettos (1981) and Falsettoland, this role was played by Michael Rupert, while Zien played Mendel, a role he reprised when these two one-act musicals were joined together and played Broadway as Falsettos in 1992. In 1998 Zien was featured in another Finn musical when he played a children's show host called Mr. Bungee, dressed as a frog, in A New Brain Off-Broadway at the Lincoln Center Mitzi E. Newhouse Theater. He received a 1999 Drama Desk Award nomination for Outstanding Featured Actor in a Musical for this role.

He appeared in the Off-Broadway play by Wendy Wasserstein Isn't It Romantic in a Playwrights Horizons production at the Lucille Lortel Theatre, opening in June 1984. He was nominated for the 1984 Drama Desk Award, Featured Actor in a Play. He originated the role of the Baker in Into the Woods in 1986 at the Old Globe Theatre in San Diego and on Broadway in 1987.

In the 1990s, Zien was part of the ensemble on the CBS sitcom Almost Perfect (1995-96), playing neurotic Screenwriter Gary Karp. Almost Perfect was cancelled shortly into its second season, but the series sustained its loyal following via reruns on USA Network not long after. Zien would return to regular roles in daytime drama, first on Guiding Light in early 1999, and by that summer, as newspaper reporter Donald Steele on All My Children, a role that lasted until 2001.

From 1999 to 2000, Zien had a recurring guest role on the CBS primetime drama Now and Again as Gerald Misenbach He has appeared repeatedly as Attorney Cromwell on Law & Order. During the 2002-03 season, Zien was the announcer on daytime's The Caroline Rhea Show, which was based out of New York (on the former Rosie O'Donnell Show set). In 2006, he appeared in the critically acclaimed film United 93 as Mark Rothenberg. He played Dr. Marsh in the vampire comedy film Rosencrantz and Guildenstern are Undead. He provided the voice of the title character in Marvel Comics' Howard the Duck.

In 2005, Zien played the part of Goran in Chitty Chitty Bang Bang on Broadway.

From April 1 to June 19, 2011, Zien appeared in the Roundabout Theatre Company's production of The People in the Picture, which played at Studio 54 on Broadway.

Zien appeared in a revival of Into the Woods at the Delacourt Theater in Central Park from August 9 to September 1, 2012. He played the Mysterious Man, the father of the character he had originated in the original production 26 years prior.

Zien appears in the new Broadway musical It Shoulda Been You at the Brooks Atkinson Theatre as Murray with music by Barbara Anselmi, book and lyrics by Brian Hargrove. The show, marking the directorial debut of David Hyde Pierce, stars Tyne Daly, Harriet Harris, Sierra Boggess, and David Burtka, and opened in previews March 17, 2015, officially on April 14.