Johnston turned pro in 1978-79 with the Fort Worth Texans. In 1979-80 he was moved to the Indianapolis Checkers and was called up for four games with the Islanders. Unfortunately for Johnston, the Islanders were just the NHL's most powerful club and winning the first of four straight Stanley Cups. It was an extremely tough lineup to crack.
He played four seasons for the Checkers, winning the Adams Cup in the last two. Johnston retired in 1983.