He was born in Phoenixville, Pennsylvania. By his early twenties, he had become a worker on a ranch in Tucson, Arizona, then joining the military during World War II. In 1947, he married Jane Gilkyson and moved to California, deciding to pursue a career as a folk singer. In the 1950s, he formed a group called The Easy Riders with Richard Dehr and Frank Miller. The group had a major hit with "Marianne," both written and performed by the three. The three also wrote "Memories Are Made of This", which became a popular song in several versions, including an adaptation for the 1956 Hungarian Revolution.
In the 1960s, he left the group to work for the Walt Disney Studios, writing music both for movies and the television series The Wonderful World of Disney. In 1968 he was nominated for an Academy Award for "The Bare Necessities" from the movie The Jungle Book.
He died in Austin, Texas, while visiting family.
Guitarist Tony Gilkyson, formerly of the band X, and Austin singer-songwriter Eliza Gilkyson are Terry Gilkyson's son and daughter. His other daughter Nancy has served for 20 years as a Vice President at Warner Brothers Records.