Canada League1 Ontario

League1 Ontario was founded on November 15, 2013, in an announcement by the Ontario Soccer Association (OSA) that it would pilot the semi-professional league in 2014 and 2015 as a key pillar of long-term player development in Canada. The league would be administered by DG Sports, who also operate the province's amateur Ontario Soccer League, with Dino Rossi serving as commissioner. OSA President Ron Smale stated that the league's core group of players are to consist of U-23s, with League1 complementing the newly formed Ontario Player Development League (OPDL) elite youth league as a pathway for professional player development.

On April 8, 2014, the OSA revealed its plans for the inaugural season of League1 which would begin during the final weekend in May 2014. The season featured 10 teams, chosen through a standards-based application process, which were: ANB Futbol, Durham Power FC, Internacional de Toronto, Kingston Cataraqui Clippers, Master's FA, Sigma FC, Toronto FC Academy, Vaughan Azzurri, Windsor Stars and Woodbridge Strikers. The league champion would face the champion of the Première ligue de soccer du Québec in the Inter-Provincial Cup to determine the national Division III champion.

Dylan Sacramento of Toronto FC Academy scored the first ever goal in the league with a 10th-minute strike against Vaughan Azzurri. In the same game, Mateo Restrepo received the league's first red card. On July 22, 2014, the league and the Ontario Soccer Association announced the termination of Internacional de Toronto's license agreement due to "failure to comply with agreed-upon league standards", with league matches rescheduled for the season to accommodate the change. Toronto FC Academy were crowned the inaugural league champions on October 4, 2014, after defeating the Cataraqui Clippers 3–1 to secure the top place in the regular season standings. Vaughan Azzurri and Sigma FC contested the inaugural League1 Cup on October 19, 2014, at BMO Field, with the Azzurri winning the single-game cup final 2–1 to be crowned champions.

As the number of teams in the league continued to grow through expansion, the league introduced a two-conference format with the winner of each conference facing off in a championship match. After the 2016 season, the Inter-Provincial Cup was cancelled, with the winners of League1 Ontario and the PLSQ instead advancing to the national Canadian Championship the following season, beginning in 2018.

In 2018, the league returned to a single division, introducing playoffs for the top finishers of the league to decide the league champion. The League Cup tournament was eliminated the following season.

On November 14, 2018, the Canadian Premier League announced its purchase of League1 Ontario. According to L1O commissioner Dino Rossi, L1O would serve as "CPL's official development league."

Due to restrictions associated with the COVID-19 pandemic, the league cancelled the 2020 season and delayed the start of the 2021 season. In 2022, L1O joined League1 Canada as a founding member. League1 Canada is an alliance of soccer leagues that operate at the pro-am level.

In 2024, League1 Ontario restructured from a single league into a three-tier competition. First announced in January 2022, existing teams were divided into the top two tiers (Premier and Championship) and the lowest tier (League2) was created as an entry point for expansion clubs to the L1O system. The competition adopted promotion and relegation for clubs to move between the tiers. The 2024 season also saw the return of the L1 Cup, a league cup knockout tournament which features teams from all three tiers of the L1O system.