He was 62.
The native of Orangeville, Ont., took a leave of absence from the Rock on Nov. 13, 2014, after he fell ill while on a family vacation. Team owner Jamie Dawick became interim GM to allow Sanderson to focus on recovery.
"Lost one of my best friends today. Things just won't be the same without him. RIP T. Thoughts and prayers with the entire Sanderson family," Dawick tweeted Thursday night.
George Daniel, commissioner of the National Lacrosse League, also tweeted his condolences to the Sanderson family "on behalf of the entire NLL."
"Terry Sanderson was a giant in the sport. The impact of his loss is immeasurable. We are all truly saddened by this news," Daniel wrote.
Sanderson was a key part of several NLL teams.
His first Champion's Cup came in 2005, when as general manager of the Rock he engineered a deal with the San Jose Stealth that brought his son Josh Sanderson and nephew Phil Sanderson, among others, to Toronto. Josh set the then single-season assists record (71) as the Rock won their fifth title in seven years.
The next season, however, Toronto went 8-8 and Sanderson was fired as the Rock's GM.
He then became an assistant coach with the Philadelphia Wings in 2007, working for his brother Lindsay who was the team's GM and head coach. Both were relieved of their coaching duties at the end of the season, allowing Terry to become defensive coach for the Calgary Roughnecks.
In 2009, Sanderson was behind the bench with the Roughnecks when Calgary won its second Champion's Cup. As with Sanderson's first NLL title, he shared the championship with his son Josh, who had five goals and 14 assists in the playoffs for the Roughnecks.
That summer, Terry Sanderson returned to Toronto to become GM and assistant coach for the Rock.
Sanderson won his third Champion's Cup in 2011 with Toronto, with nephew Phil again on the Rock's roster.
Terry Sanderson's success in the NLL did not come immediately. He began his NLL coaching career in 2000 with the Albany Attack. After two seasons in Albany, N.Y., he resigned to become head coach and GM of the expansion Montreal Express. The Express folded after one season.
He then moved on to the Ottawa Rebels, replacing his brother Lindsay as head coach in 2002. Ottawa folded after the 2003 season, leaving Terry to join the Rock's front office for the first time.