Known for his big left-hook and his imposing size, the tall, lean Cooney had his first paid fight on February 15, 1977, beating Billy Jackson by a knockout in one round. Nine wins followed and Cooney gained attention as a future contender. Although his opponents were carefully chosen. He moved up a weight class and fought future world cruiserweight champion S.T. Gordon in Las Vegas, winning by a fourth round disqualification. Cooney had 11 more wins, spanning 1978 and 1979. Among those he defeated were Charlie Polite, former US heavyweight champion Eddie Lopez, and Tom Prater. These were not rated contenders however.
By 1980, Cooney was being featured on national television. Stepping up he beat one time title challengers Jimmy Young and Ron Lyle, both by 'knockout,' although the Young fight was stopped because of cuts sustained by Young. By now he was ranked number 1 by the WBC and eager for a match with champion Larry Holmes.
In 1981, he defeated former world heavyweight champion Ken Norton by a knockout just 54 seconds into the first round with a blisteringly powerful attack, which broke the record set in 1948 by Lee Savold for the quickest knockout in a main event in Madison Square Garden.
The following year, Holmes agreed to fight him. With a purse of ten million dollars for the challenger, it was the richest fight in boxing history to that time. The promotion of the fight took on racial overtones that were exaggerated by the promoters, something Cooney did not agree with. He believed that skill, not race, should determine if a boxer was good. However, if Cooney won, he would have become the first Caucasian world heavyweight champion since Swede Ingemar Johansson defeated Floyd Patterson 23 years earlier. This caused Don King to label Cooney "The Great White Hope." The bout drew attention worldwide, and Larry Holmes vs. Gerry Cooney was one of the biggest closed-circuit/pay-per-view productions in history, broadcast to over 150 countries.
Cooney fought bravely after he was knocked down briefly in the second round. Some believe he was winning until he was deducted three points for repeated low blows in round 10. But after 12 rounds, the more skillful and experienced Holmes finally wore him down. In round 13, Cooney's trainer stepped into the ring to save his fighter from further punishment.
After a long layoff, Cooney fought in September 1984, beating Phillip Brown by a 4th round knockout in Anchorage, Alaska.He fought once more that year and won, but personal problems kept him out of the ring.
Cooney was far past his prime when he made an ill advised comeback against former world heavyweight and world light heavyweight champion Michael Spinks. Boxing carefully with constant sharp counters Spinks knocked him out in round 5. Cooney's last fight was in 1990. He was knocked out in a match of the veterans in two slugging rounds by former world champion George Foreman. Cooney did stagger Foreman in the first round however, but he was simply overmatched.
Cooney compiled a professional record of 28 wins and 3 losses, with 24 knockouts. He is ranked number 53 on Ring Magazine's list of "100 Greatest Punchers of All Time".