The first thing you need to know about Jim Moore is that his memory is probably better than yours. He can recall the exact date he joined GQ as a lowly assistant (Jan. 30, 1980), what he wore (a turtleneck) and what the offices at the men’s fashion magazine looked like (“We had really nice digs, there were four desks in our section and an open floor plan”). After nearly 40 years at GQ, he’s stepping off the front lines of fashion this month, moving from creative director into a more flexible, less hands-on role, “creative director at large.”

It’s...