Doubell's first major international victory in 800 metres was at the World Student Games in Tokyo in 1967 in a time of 1:46.7. His next season (the Olympic season) was severely curtailed by Achilles' tendon injuries, and he was unable to compete for six months prior to the Olympic Games in Mexico City. Doubell, however, was able to recover in time for Mexico City and won the 800 m gold medal, passing the pre-race favourite Wilson Kiprugut of Kenya down the straight to win in a world record equalling time of 1:44.3.
Doubell also won the 800 metres gold medal at the 1969 Pacific Conference Games in a time of 1:48.0.