Born in St. Louis, Missouri, where he attended Normandy High School, Harkins began acting professionally in the mid-1950s on the Broadway stage after studying at the University of Iowa.

A life member of The Actors Studio,[1] Harkins appeared in productions of The Terrible Swift Sword, Good as Gold, and Mother Courage and Her Children, prior to making his television debut in a 1965 episode of The Trials of O'Brien.

The next year, Harkins made his film debut in The Three Sisters, opposite Shelley Winters and Geraldine Page.

During the 1980s, Harkins continued with roles in television and films appearing as a cynical lawyer in the 1981 thriller Absence of Malice, starring Paul Newman, an ill-fated real-estate agent in the 1983 horror film Amityville 3-D,1983 in The Return Of The Man From U.N.C.L.E[The Fifteen Years Later Affair] and former Communist and Cold War-era figure Whittaker Chambers in the 1984 PBS mini-series, Concealed Enemies.

In 1988, Harkins co-starred in the television adaptation of Inherit the Wind starring Jason Robards and Kirk Douglas, followed by a role in Slaves of New York.