Alliance United joined the men's division of
League1 Ontario for the
2018 season. They played their inaugural match on April 29, 2018 against
North Mississauga SC at home, which they lost by a score of 1–0. They finished in 7th place in their debut season. They improved to a 3rd-place finish in the
2019 season, but had a very poor showing in the playoffs where they lost 15–0 to
FC London on aggregate in the semi-finals over the two-game series, due to being unable to field many of their best players, who missed the series due to college and university commitments, thereby having to rely on many youth players instead. In
2019, the club added a team in the
League1 Ontario women's division. In their debut season, they finished in a three-way tie for fifth, but did not play in 2020 or 2021 due to the
COVID-19 pandemic. In addition, they advanced to the
League1 Canada Women's Interprovincial Championship for 2022, as NDC Ontario was unable to compete due to player unavailability, before defeating
League1 British Columbia side
Varsity FC in the third-place match in another penalty shootout. In 2023, after finishing 4th in the regular season, they won the women's league championship defeating
NDC Ontario in the playoff final. In 2023, they formed an affiliation with
Canadian Premier League club
York United FC, which enables York United to send players to Alliance on short-term loans. On July 29, 2023, they recorded their largest victory, defeating
Unionville Milliken SC by a score of 13-1. In 2024, the men's second team won the inaugural third tier League2 Ontario title, earning promotion to the second tier League1 Ontario Championship. However, they chose to decline the promotion to remain in League2 Ontario for 2025. In January 2026, it was announced that the club sold their League1 Ontario license to
International FC, ending their participation in League1 Onatrio. == Seasons ==