Lyndeck, a former college professor, spent well over a dozen years in stock and regional theater roles before making his Broadway debut in the original production of 1776. After originating the role of John Witherspoon, Lyndeck went on to play Stephen Hopkins, Dr. Lyman Hall, and Charles Thomson before ending up on tour as John Dickinson. His other Broadway credits include Mrs. Warren's Profession, Piaf: A Remembrance, A Doll's Life, Merlin, and Into the Woods. His best-known role is the evil Judge Turpin in the original production of Stephen Sondheim's Sweeney Todd, which he also repeated for the first national tour, the 1980 television broadcast, and a 1994 revival at the North Shore Music Theatre. Lyndeck earned a Drama-Logue Award for his performance in the tour's Los Angeles berth.
Lyndeck works frequently in Pennsylvania regional theater. He is a fixture at the Pittsburgh Civic Light Opera — most notably as Scrooge in their annual musical version of A Christmas Carol, a role he has performed every year since 1992.
Lyndeck appears occasionally on film and television. He was a contract player on a now-defunct soap opera, The Doctors, and has had guest roles in series like Ed, The Cosby Show, and Law & Order: Special Victims Unit. Lyndeck's best-known film role is as the marijuana-smoking grandfather in Road Trip; he is also known for his appearance in Adam Sandler's Big Daddy and his most recent appearance in The Notorious Bettie Page. He has also done voiceovers for numerous television and film documentaries.