Born on the Red Pheasant Reserve near Battleford in December 1887, Alex Decoteau attended the Battleford Indian Industrial School, where he excelled in soccer and running. His first competitive race was a one-miler at Fort Saskatchewan on May 24, 1909, where he came in second. On July 1, 1909, he set a new western Canadian record (27.45.2) for the five-mile Mayberry in Lloydminster; he also won the Hon. C.W. Cross Challenge Cup Race (26.34.4) in Edmonton later that summer. Decoteau won a ten-mile race in Fort Saskatchewan in 1910 - eight minutes faster than his nearest competitor - and then entered and won the half-mile, one-mile, two-mile and five-mile races on July 1 in Lethbridge. Decoteau moved to Edmonton to work in his brother-in-law's machine shop, and raced competitively with the Irish-Canadian Athletic Club. In 1911 he joined the Edmonton City Police force as the first Aboriginal to serve on the force, and was one of the first motorcycle policemen in the city. Decoteau was promoted to desk sergeant of the South Side Police Station in 1914, and continued to run competitively under the colours of the Edmonton Police Association. On Christmas day in 1910 Decoteau set the record for the Calgary Herald 's annual 6.33 mile race (on a snow-covered course) at 34 minutes, 19 seconds. He continued to win this race every time he entered it; with his record unbeaten, the cup was permanently presented to him in 1915.
Decoteau tried out for the Canadian Olympic team in 1912 (10,000-metre), but withdrew because of a leg cramp; he competed in the 5,000-metre race, winning easily but with a time not considered good enough for the Olympics. The Canadian Olympic Committee ordered a race between Decoteau and the favourite from Vancouver; Decoteau set a new Dominion record at 15.27.4 and became the only athlete from Alberta to participate in the games. In Stockholm, Decoteau placed second in the 5,000-metre and sixth in the final, and was awarded an Olympic Merit Diploma and a medal for his performance. In 1916 Decoteau resigned from the police force and joined the Canadian Army, winning several races while stationed in England. He was killed near Passchendaele Ridge in France on October 30, 1917. Decoteau was elected to the Edmonton City Police Hall of Fame (where his many of his medals, cups, and photos are displayed), and has been inducted into both the Alberta Sports Hall of Fame and the Saskatchewan Indian First Nations Sports Hall of Fame.