Alan joined the Ontario Provincial Police in November 1966 and was posted to Kemptville Detachment.

On March 31, 1975 Constable Thompson was in pursuit of a speeding vehicle which he followed over the train tracks just south of Kemptville. A freight train on a regular run was just approaching the unmarked crossing. The vehicle being pursued passed over the crossing, however, the cruiser carrying Constable Thompson was hit by the passing train and dragged for several yards.

Alan Thompson was gravely injured and trapped in his vehicle for some time. Alan did not survive his injuries.

Alan Thompson was survived by his wife and their two children. He was thirty-five years of age.

On April 24, 2015, a little more than forty years after his death, Constable Thompson was honoured with a permanent memorial, a bridge dedicated in his honour at Highway 416 Canadian Pacific Railway overpass. Signs have been erected on both sides of the bridge in Kemptville for all motorists to see while passing through the area. This bridge dedication and all others like it, act as a reminder of the sacrifices made by the men and women of the Ontario Provincial Police and honour their dedication in serving the people of the Province of Ontario.