Jackson Gillis, the middle son of three brothers, was born to Ridgway M. Gillis, a civil engineer for the Washington State Highway Department, and Marjorie Lyman. In the late 1920s, the Gillis family relocated to California, initially settling in Fresno before making Sacramento their home.
Growing up in Sacramento, Jackson attended and graduated from Stanford University. In July 1941, he married stage actress Patricia Cassidy.
