After winning the AAU championships in 400 m hurdles in 1939, Cochran was selected to the 1940 US Olympic team to run 400 m flat, 400 m hurdles and 4x400 m relay race. But when the 1940 Summer Olympics were cancelled due to the World War II, Cochran entered the V-7 Navy officer training course in 1942, and went to Miami for training in the Navy's Sub Chaser Training School. He served in the Pacific during the war and attended the University of Southern California in pursuit of graduate degrees in physiology after the war.
Cochran took up athletics again at USC and won his second AAU title in 400 m hurdles in 1948, thus qualifying to the Olympics. In the Olympic final Duncan White from Ceylon went off at a terrific pace, but by half distance Cochran was ahead. He won by a huge margin, beating second-placed White by 0.7 seconds. Cochran won his second gold medal as he ran the third leg of the 4x400 m relay for the winning USA team.