Subsequent Broadway appearances included Neil Simon's The Gingerbread Lady, with Maureen Stapleton, David Storey's The Changing Room, David Rabe's Sticks and Bones, and the 1974 revival of Tennessee Williams' Cat on a Hot Tin Roof starring Elizabeth Ashley, Fred Gwynne, and Keir Dullea.

Notable Off-Broadway appearances include Colette starring Zoe Caldwell, and Rubbers directed by Alan Arkin.

Moving to Los Angeles in 1976 Siebert made his first feature film appearance in the horror cult classic Blue Sunshine.

He then began appearing as a guest artist on numerous television programs such as One Day at a Time, The Blue Knight, The Rockford Files, Murder, She Wrote and Mancuso, F.B.I.

Film roles throughout the mid to late 1970s included ...And Justice for All with Al Pacino, Michael Crichton's Coma, All Night Long with Barbra Streisand and Gene Hackman, and White Water Summer with Sean Astin and Kevin Bacon.