Johnstone played major junior with the Medicine Hat Tigers of the Western Canada Hockey League from 1971 to 1974. After two 100 point seasons in a row with Medicine Hat, Johnstone was drafted 104th overall by the New York Rangers in the 1974 NHL Amateur Draft, and 78th overall by the Michigan Stags in the 1974 WHA Amateur Draft. He played 23 games for the Stags in 1974–75 and played in the American Hockey League for the Providence Reds. Johnstone started his National Hockey League career in 1975–76, scoring 3 points in 10 games. Johnstone played for the Rangers until the end of the 1982–83 season, before being traded to the Detroit Red Wings along with Ron Duguay and Eddie Mio for Mike Blaisdell, Willie Huber, and Mark Osborne. He played for the Red Wings and their AHL minor league affiliate, the Adirondack Red Wings before retiring in 1987.