Born at Enfield in Saint Mary Parish, Jamaica, Cowans moved to England with his family when he was eleven. He played County Championship cricket for Middlesex and then Hampshire, winning three Championships and four limited-overs titles (all with Middlesex) in his fifteen seasons. On his first tour overseas, on England's defence of the Ashes in Australia in 1982/83, he often struggled, was wayward in line and length, and was underbowled by captain, Bob Willis, until the crucial Fourth Test at Melbourne, a match England had to win if they had any hope of retaining the Ashes they won at home in 1981.

Cowans played the game of his life at the MCG in 1983, where he took a match-winning 6 for 77, following his first innings 2 for 69 (which included the first ball scalp of Greg Chappell), in England's dramatic three run victory. For these five days, Cowans was the star of English cricket, and had sent the series to Sydney for the deciding Fifth Test, which ended in a draw.

Cowans was nicknamed 'Flash' as a result of being able to bowl a cricket ball at almost 100 mph.^{[citation needed]}

He was Richard Hadlee's 200th Test wicket, of which Hadlee said: "I would have preferred it to have been Gower, Randall, or Lamb, but Flash will do in the circumstances!"^{[citation needed]}

Cowans is currently a regular commentator on Sky News and BBC News 24, and has his own sports promotions company.