He played hockey for the Brandon University and Acadia University and then became a coach at the age of 23. He at first coached Camrose College in Alberta and then was an assistant coach at the University of Calgary. In 1989 he took over as head coach at the University of New Brunswick, a job that lasted until 1994.
In 1994 he became general manager and associate coach of the Canadian national men's hockey team. In 1998 he became the head coach for one season. He then spent six seasons as, at first, an assistant coach and then as an associate coach with the Vancouver Canucks. After that he was an associate coach of the Los Angeles Kings. In 2008 he started his present job in Portland.
Besides all this, Johnston has extensive experience in international play. He won the gold medal at the 1997 and the 2007 World Championships. He also won the silver in 1996 and the bronze in 1995. In 1994 and 1995 Johnston won the gold at the World Junior Ice Hockey Championships.
On June 25, 2014, Johnston was named the new head coach of the Pittsburgh Penguins.