Johnston won the Brisbane Rugby League premiership with Souths in 1981.
Johnstone played in the 1982 Brisbane Rugby League grand final for Souths against Wynnum-Manly. He played at hooker in Canterbury's 1985 Grand Final victory.
Johnstone was also appointed the inaugural captain of the Gold Coast-Tweed Giants. He later became the club's recruitment manager.
After retirement, Johnstone has become a highly regarded Strength and Conditioning Coach in the NRL. He worked with the North Queensland Cowboys from 2002 to 2006 before moving to the newly established Gold Coast Titans. The North Queensland club however have again offered Billy a deal he couldn't turn down. In July 2008, he was granted a release from the Gold Coast Titans and has signed a three-year deal with the North Queensland Cowboys, beginning in 2009. Billy Jonstone also is currently operating as a personal trainer from a local gym in Townsville, Queensland named 'The Zoo Health & Fitness, where he trains intermediate to advanced members and uses his Strength & Conditioning knowledge to good value.http://www.thezoohf.com.au/zoo/staffprofiles.html