Hagan was born Janet Kay Ruthven in Shelby, North Carolina, the daughter of Jeanette (née Chiles), a homemaker, and Josie Perry "Joe" Ruthven, a tire salesman. Her uncle (mother's brother) was the Lakeland native and U.S. Senator Lawton Chiles (D-Fla.), who later became Florida Governor following his service in the U.S. Senate. Both Hagan's father and her brother served in the U.S. Navy. She spent most of her childhood in Lakeland, Florida, and graduated from Lakeland Senior High School in 1971.