Frederick L. Bennett was born on September 9, 1915 in Wilson Mills, Oxford County, Maine. He was the first and only child of Frederick John Bennett and Sadie Johnson Blakely Bennett. Bennett's father was most likely from Magalloway Plantation, Oxford County, Maine, and his mother was from either Maine or New Hampshire. His parents had wed about three years before his birth. His father was employed for the Brown Company. His grandparents on his father's side were named Elias Bennett and Lucy Alice. Bennett and Alice were wed around 1878 and Frederick John Bennett was their only child.