His first film as cinematographer was the Western Blood Test from 1923. Together with cinematographer Ernest Haller, he won the Oscar for Gone with the Wind. He received the award for his color photographs which he had made in the function of serving as an associate photographer. In the same year he was nominated along with Bert Glennon for their work on drums at the Mohawk for an Oscar. For the film, King of the Toreros in 1941 he received his second Oscar with his colleague Ernest Palmer.
From the mid- 1950s Rennahan worked mainly for television. He was involved in several television series, including 1959 to 1963 to At the foot of the Blue Mountains. His last camera work dates from the year 1973. Overall, he had been involved in over 130 productions.
Better known directors, with whom he collaborated more than once, are Cecil B. DeMille and Michael Curtiz
From 1950 to 1951, as well as in the years 1965 and 1966 he was president of the American Society of Cinematographers.
He has a star on the Walk of Fame since 1978.