William Mandella is the major character in Joe Haldeman's Forever War series, the protogonist of The Forever War and Forever Free. He was born on Earth sometime in the early 1980s, and majors in physics in college. Following the start of the war with the Taurans, he elects to join the newly created United Nations task force (as the team physics specialist).

It is through his eyes that the reader experiences both the war against the alien Taurans and the changes in society and technology on Earth during the thousand-year conflict (made possible by time dilation, a relativistic consequence of faster-than-light travel between battles).

During the war, he falls in love with Marygay Potter. Upon the conclusion of the war, he marries her and the move to the distant planet "MF" to raise their two children.