Millen switched to the IMSA Camel GT series in 1990. He was the 1990 24 Hours of Le Mans Rookie of the Year. He won three races and took five pole positions in 1990. He had seven poles and five wins in 1991. He also drove for Allan Moffat Racing in the 1991 Tooheys 1000 at Bathurst, partnering Gary Brabham in a Ford Sierra RS500 but the car did not finish the race. Millen expressed interest in returning by no offer was forthcoming with the Moffat team seeing the return of the cars builder Ruedi Eggenberger in 1992 along with his ace driver Klaus Niedzwiedz.