As of 1933, Bennett was a member of the St. Lambert Canoe Club and the Excel Boating Club in Longueuil, and won major honours in numerous competitions. Although he qualified for the 1940 Olympics, he had to wait until the end of the Second World War before he was able to devote himself to being an Olympic athlete. In the meantime, he enlisted in the artillery and remained in Europe throughout the war.
On his return, he began working for Bell Canada and again resumed his favourite sport of canoeing. On 3 Aug 1947, he was crowned Canadian champion in the C-1 and C-2 1000 metres with Harry Poulton in the Canadian Canoe Association regattas. Armed with these outstanding results, he received his pass for the London Olympics that would take place the following year.
In London, he raced in the C-1 event. After an early race, the paddler decided to take a nap before the finals. When he woke up, his competitors were already in place at the starting line. Realizing that the Canadian was not in line beside him, a British opponent deliberately delayed the race by making two false starts. During this time, Bennett had to paddle the full distance to take up his position. Having made such an effort to take part, the Canadian fought a tight battle and finished in second place. He was beaten by a Czech opponent whose canoe had a curved keel that made steering much easier. The Canadian team protested on the grounds that the canoe gave the Czech team an advantage, but in vain. The following year the International Canoe Federation revised its rules to prohibit canoes with curved keels.
In the C-2 event, Bennett and Poulton placed fourth in the 1000 metre event.
With one silver medal and two bronze, Canada made a poor showing in the 1948 Summer Olympics.
Doug Bennett ended his canoeing career in 1949 when he won the Canadian C-4 championship.
In 1947, he bested Maurice Richard when the Montreal Sportsman's Association named him athlete of the year. Forty years later, in 1987, Douglas Bennett was inducted into the St-Lambert Hall of Fame. In 2000, he was welcomed into the Canadian Olympic Hall of Fame, and he was inducted into the Quebec Sports Hall of Fame in 2003.