Upon retiring from the ice, Popein entered coaching. He served as the bench boss of the Omaha Knights of the Central Hockey League, then as an assistant coach with the Rangers; he briefly was the interim head coach of this club during the 1973–74 season, coaching for 41 games. He was fired on January 11, 1974 after suspending Rod Gilbert for a game due to missing a team meeting: the meeting time had been moved but Popein had not informed Gilbert, though he upheld the suspension. With this move the rest of the Rangers lost confidence in Popein, forcing him out.
