Jackson DeForest Kelley was born on January 20, 1920, in Atlanta, Georgia, to Clora (née Casey) and Ernest David Kelley, a Baptist minister of Irish and Southern ancestry. He was named after the pioneering electronics engineer Lee De Forest, and later named his Star Trek character's father "David" after his own. Kelley had an older brother, Ernest Casey Kelley. As a child, he often played outside for hours at a time and was immersed in his father's mission in Conyers, where he promised his father that failure would mean "wreck and ruin".
Kelley was introduced to his musical talents at a young age and often sang solo in morning church services. His musical abilities led to an appearance on the radio station WSB AM in Atlanta, Georgia, and he eventually won an engagement with Lew Forbes and his orchestra at the Paramount Theater. In 1934, the family left Conyers for the community of Decatur, where Kelley attended Decatur Boys High School. He played on the Decatur Bantams baseball team, and also played football and other sports.
Before graduating in 1938, Kelley got a job as a drugstore car hop and spent his weekends working in the local theaters. During World War II, Kelley served as an enlisted man in the United States Army Air Forces between March 10, 1943, and January 28, 1946, assigned to the First Motion Picture Unit. After the war, Kelley decided to pursue an acting career and relocate to southern California permanently, living for a time with his uncle Casey. He worked as an usher in a local theater in order to earn enough money for the move. Kelley's mother encouraged her son in his new career goal, but his father disliked the idea.
While in California, Kelley was spotted by a Paramount Pictures scout while doing a United States Navy training film.
