Geoff Johnstone was a driving force behind the formation of the Conestoga College soccer program. His coaching career at Conestoga includes coaching the men's outdoor team (1968-2008), women's outdoor team (1984-2000), men's indoor team (1986-2009) and the women's indoor team (1990-2000).

During his time as the women's outdoor coach his team claimed the OCAA championship in the 1991-92 and the 1992-93 seasons. With the men's outdoor team Geoff captured gold in the 1979-1980 seasons. He also won OCAA gold medals for women’s indoor team in the 1995-96 season and for the men's indoor team in the 1988-89 and the 1991-92 seasons. Johnstone was named OCAA Coach of the Year in 1987 and 1996 for the men's team and in 1996 for the women's.

In total, Johnstone collected a total of 26 OCAA championship medals in his 38 years of coaching. He was known for being a great leader in the OCAA and making decisions that would benefit soccer at the college level. Geoff is the longest serving coach in OCAA history.