The league championship team each year goes on to play against the champions of the Western Lacrosse Association for the Mann Cup in September.

In its early days, it had competition from the semi-professional National Lacrosse Union that ran from around 1880 until around 1920 with teams in the Toronto, Ottawa, and Montreal markets.

In the early 20th century the CLA had another rival league, the Dominion Lacrosse Association in much of the same markets as the NLU.

The Westonmen were met by the Winnipeg Tigers, whom they defeated 11-3 to become the first Mann Cup champion of the playdown era.

The Westonmen were then forced to face the Brampton Excelsiors in the Ontario final who beat them 10-goals-to-6 in a two-game series.

The Mimico Mountaineers would defeat the Fergus Thistles 2-games-to-1 to become the first Ontario Senior A box lacrosse champions.

In 1951, the core of the 1950 Mann Cup winning Owen Sound Crescents, including star goalie Lloyd "Moon" Wootton transferred to the Peterborough Trailermen.

Peterborough's star scorer Bob Allan left a BC team without receiving a release to come play for the Trailermen.

Peterborough refused to take the floor for the first game without Allan and the CLA kicked them out of the playoffs and suspended every player on the team for five seasons.