After several disappointments, he earned a job as Paul Newman's understudy in the Broadway play Sweet Bird of Youth in 1959.

Steady television guest appearances followed, including a role on NBC's series The Tall Man.

Harper appeared in soap operas, including CBS's series Where the Heart Is and Love of Life.

He appeared as a regular performer for several television series, including Planet of the Apes, and as Uncle Jack for the third season of Land of the Lost.

His movie credits included roles in Below Utopia (1997), The Odd Couple II (1998), Freedom Strike (1998), Glass Trap (2005) and The Poughkeepsie Tapes (2007).