Playing in the waning days of the period where teenagers were signed to junior league contracts by NHL teams, Sheppard played his junior hockey for the Estevan Bruins of the Western Hockey League, competing for the Memorial Cup in two seasons and being the star of his team's Memorial Cup drive in 1968.
In 1969 Sheppard began a three-year pro apprenticeship with the Oklahoma City Blazers of the Central Hockey League, becoming a Second Team All-Star in 1971.
The following season, he was named the league's most valuable player and remains the all-time leading career scorer of the franchise.
Joining the defending Stanley Cup champion Bruins in 1972 as a result of the parent club losing players to expansion and the new World Hockey Association, Sheppard – showing both scoring prowess and exemplary defensive and penalty killing skills – had a fine rookie season, finishing sixth in balloting for rookie of the year honors.
At first holding out due to dissatisfaction at the deal, he eventually joined the team in late November 1978, although his days as a scorer were behind him.