The Mimico Lacrosse Club was established in 1890 as a field lacrosse organization. In 1931, when the Canadian Lacrosse Association chose the box lacrosse game as its new form of play, the Mimico Lacrosse Club built an outdoor box at the corner of Church Street (Royal York) and Drummond Street, where the game has been played continuously until today. The Mountaineers won the Mann Cup in 1932 and 1942, and Eastern Canadian Senior titles in 1943 and 1947. In World War II, Conn Smythe's 30th Light Anti-aircraft Battery, dubbed "The Sportsmen's Battery", included every member of the Mimico Mountaineers who won the 1942 Mann Cup. Minto Cup victories were also recorded in 1938 and 1951, with Eastern Canadian championships in 1941 and 1946.
Mountaineers player 2014
Since expansion in 1993 with the Nepean Knights, the Mimico Mountaineers have never really wavered from "average". In recent years there has been improvement, but overall the Mountaineers have always been around the .500 mark and usually a quick exit from the playoffs. In the past four seasons,[when?] this trend is starting to take an up swing—but not very quickly. The 2006 season was a somewhat landmark year for the Mountaineers, as they made the playoff semi-finals. After defeating the Clarington Green Gaels 3 games to 1 and the Barrie Tornado in the quarter-finals 3 games to 1, the Mountaineers quietly slipped out of the playoffs with a 3–0 sweep by the eventual Founders Cup champions, the Oakville Buzz.
The Mountaineer line-up for the 2006 season included 10 rookies and the team went the furthest it has even been into the playoffs. The 2007 season ended when the Mountaineers were eliminated in five games of round two of the playoffs by Halton Hills.