After sitting out the 1978 season, Gregg returned to coaching in 1979 with the Canadian Football League's Toronto Argonauts. In 1980, he became the head coach of the Cincinnati Bengals for four seasons through 1983. His most successful season as a head coach was in 1981, when he led the Bengals to a 12–4 regular season record. They defeated the San Diego Chargers 27–7 in the AFC championship game (known as the Freezer Bowl), earning them a trip to Super Bowl XVI, where they lost to the San Francisco 49ers, 26–21.