Clark first played lacrosse for Ajax at the age of nine. He played five years of Junior lacrosse, three and one half years for Scarborough and one and one half years for Whitby. Clark was called up in 1990 to help Brooklin win the Mann Cup, and was drafted by the Redmen in 1991, scoring 55 goals and 126 assists plus 467 PIM in 190 games, including a brief stint for Brantford in the "old" NLL in 1991. Glenn was the Defensive Player of the Year for Brooklin all five of the summers in which he played the full schedule.
NLL career
Clark was drafted by the Ontario Raiders, and played in 1998, the Raiders' only season. The next year, the Raiders moved to Toronto, becoming the Toronto Rock. Clark played seven seasons with the Rock, helping lead them to five NLL championships. Clark appeared in two All-Star Games as a member of the Rock, in 2004 and 2005. After the 2005 season, Clark signed as a free agent with the Philadelphia Wings. He played the 2006 season as a member of the Wings and was selected to the All-Star team for a third time. After the 2006 season, Clark was offered the position of head coach of the Toronto Rock, and immediately retired from playing to take the job. His debut season with the Rock was not as successful as Clark was hoping for; the Rock opened the season with three straight losses before climbing to a final 6-10 record. This was good enough to make the playoffs, but the Rock were beaten 10-6 by the eventual Champion's Cup-winning Rochester Knighthawks. On January 11, 2008, the Minnesota Swarm defeated the Toronto Rock 17-16 in overtime. After the game, Clark was involved in an altercation with Minnesota forward Sean Pollock outside the Rock dressing room. Clark was later charged with assault by Toronto Police. On January 17, the NLL announced that Clark was suspended indefinitely. The charges against Clark were eventually dropped, and the suspension was lifted on February 13. The Rock won their 2009 season opener against the New York Titans, but after they lost their next two games, scoring a total of only 15 goals, Clark was fired on January 20, 2009 along with assistant coach Terry Bullen. Assistant coachJim Veltman was retained by the Rock in an advisory position. Clark was replaced with Jamie Batley. On June 24, 2015 Clark was named the second head coach of the New England Black Wolves, joined by assistants Jim Veltman and Tracey Kelusky.
