McCowen was born in Dublin, Ireland. He was educated at Cheltenham College and Cambridge University. In 1932 he was a member of the winning Cambridge boat in the Boat Race. The 1932 crew won the Grand Challenge Cup at Henley Royal Regatta rowing as Leander Club. They were subsequently chosen to represent Great Britain at the 1932 Summer Olympics in Los Angeles, where they came fourth in the eights.
During World War II McCowen joined the Royal Navy Volunteer Reserve and served in the coastal forces and on motor torpedo boats. He was awarded the DSO and DSC for actions in 1944, and received the surrender of the first surface craft (2 E-boats) from Rear Admiral Karl Bruning on 13 May 1945.
- List of Cambridge University Boat Race crews
- Motor Torpedo Boat
- E-boat – includes a photo of one of the E-boats surrendering
- Normandy landings