McLellan started his playing career with the Saskatoon Blades of the WHL from 1983 to 1987. In the 1986 NHL Entry Draft, he was drafted by the New York Islanders in the fifth round. He played a total of five games at the NHL level, spending most of two seasons with the Islanders' American Hockey League affiliate, the Springfield Indians. However, recurring shoulder injuries dating back to his junior hockey days ended McLellan's North American playing career after the 1988–89 season. He returned home to study at the University of Saskatchewan for a year, before resuming his playing career for SIJ Utrecht of the Eredivisie in the Netherlands. During his three seasons there, the team hired a new coach, who moved in with McLellan and gave him coaching responsibilities, which McLellan recognizes as the reason he became interested in coaching.
Following his stint as a player-coach with SIJ Utrecht, McLellan returned to Canada in 1992. He went into full-time coaching in 1993, being hired as the coach of the North Battleford North Stars of the Saskatchewan Junior Hockey League. In 1994, McLellan was hired as the coach and general manager of the Swift Current Broncos of the Western Hockey League. In his six seasons with Swift Current, the Broncos qualified for the WHL playoffs in all seasons. McLellan himself was named WHL Executive of the Year in 1997 and Coach of the Year in 2000.
Following his successes at the junior level, McLellan was hired by the expansion Minnesota Wild to coach their minor league affiliate, the Cleveland Lumberjacks of the International Hockey League. After the IHL folded in 2001, McLellan and his staff were transferred to the Wild's new minor league affiliate, the Houston Aeros of the American Hockey League. As coach of the Aeros, McLellan led Houston to the Calder Cup in 2003.
In 2005, Mike Babcock selected McLellan to serve as his assistant with the Detroit Red Wings. In Detroit, McLellan was tasked with handling the Red Wings' forwards and managing the team's power play, as well as reporting player performance to head coach Babcock. Under his watch, the Red Wings had the top-ranked power play in the NHL, finishing first in power play efficiency in 2005–06 and third in 2007–08. McLellan also achieved his first Stanley Cup with the Red Wings in 2007–08.
On June 11, 2008, the San Jose Sharks hired McLellan to become their new head coach, replacing Ron Wilson. He would end up the head coach for the Western Conference All-Star team, and lead the Sharks to their first Presidents' Trophy with an NHL-leading 117 points to finish the regular season, and finished third in voting for that season's Jack Adams Award, behind winner Claude Julien and Andy Murray.
|Team||Year||Regular season||Post season|
|SJ||2008–09||82||53||18||11||117||1st in Pacific||Lost in first round (ANA) | Series 4-2|