Cosens was born in Cayuga, Ontario. He was 23 years old and a sergeant in The Queen's Own Rifles of Canada during the Canadian Army's involvement in the Second World War.
On the night of 25-26 February 1945 at Mooshof near Uedem, Germany, Sergeant Cosens assumed command of the four survivors of his platoon whom he placed in position to give him covering fire. Running forward alone to a tank, he took up an exposed position in front of the turret and directed its fire. When a further counter-attack had been repulsed, he ordered the tank to ram some farm buildings into which the Germans attackers had retreated. He went in alone, killing several of the defenders and taking the rest prisoners. He then dealt similarly with the occupants of two more buildings, but was shot by a sniper when he went to report back to superior officers