A successful radio actor, Lovejoy was heard on the 1930s crime drama series Gang Busters. Lovejoy was a narrator (during the first season) for the show This Is Your FBI. He played the title character on the syndicated The Blue Beetle during the 1940s, and starred in the later newspaper drama series Nightbeat in the early 1950s and in episodes of Suspense in the late 1950s.
In films of the 1940s and 1950s, Lovejoy mostly played supporting roles. Appearing in movies such as Goodbye, My Fancy (1951) withJoan Crawford, and The Hitch-Hiker (1953) directed by Ida Lupino, Lovejoy was effective playing the movie's everyman in extraordinary situations. He was in several war movies, notably Joseph H. Lewis' Retreat, Hell! (1952) which portrayed the United States Marine Corps' retreat from the Chosin Reservoir (aka the Changjin Reservoir) during the Korean War. In 1951, he had the title role in I Was a Communist for the FBI with co-stars Ron Hagerthy, Paul Picerni, and Philip Carey.
Lovejoy starred in two short-run TV series, Man Against Crime and Meet McGraw. Episodes of these two series have never been released commercially on DVD or VHS and never aired on reruns. Meet McGraw episodes were screened at the Mid-Atlantic Nostalgia Convention.
Lovejoy was first married to Frances Williams (1901–59) but divorced in the late 1930s. In 1940, Lovejoy married actress Joan Banks(1918–1998), with whom he had a son and a daughter.
Among Lovejoy's last performances was with Donald May in the episode "County General" (March 18, 1962) on the ABC series Bus Stop with Marilyn Maxwell in the role of Grace Sherwood, owner of a diner in Sunrise, Colorado. That same season, he appeared on the ABC crime drama Target: The Corruptors! about the efforts of a New York City reporter to expose organized crime.
On October 2, 1962, Frank Lovejoy died of a heart attack in his sleep at his residence in New York City. His wife, Joan Banks, called for medical help after she was unable to wake him. The couple had been appearing in a New Jersey production of the Gore Vidal play The Best Man.