The Athletic Union was established on 17 April 1885 to promote sporting activities at Owens College. Previously, there had been no co-ordinating body for College sports, although sporting events had featured at the College as early as 1862. The Union originally comprised four men's clubs: Rugby Union, Lacrosse, Cricket and Tennis. The Union excluded women's clubs, and consequently a separate Women's Athletic Union (WAU) was formed in December 1900. The Unions maintained separate existences until the merger of 1981.
Although the Union did not initially attract many members, it won enthusiastic support from some academics, particularly Arthur Milnes Marshall, the professor of zoology, and Harold Dixon, professor of chemistry, who served as successive presidents of the Union. They lobbied for improved faculties, and compulsory subscription of College sports clubs to the Union. In 1898, the College acquired the Firs Estate in Fallowfield for a new sports ground, and this opened in 1900. In the same year, the Women's Athletic Union was instituted. Students in the Faculty of Technology, from 1905 based at the College of Technology (later UMIST), formed their own athletic union, which was associated with the Manchester Athletic Unions, and represented on some of their governing bodies.
In 1905, the Athletic Union officially adopted maroon and white as the University colours, with the Women's Union adopting maroon, green and white for its colours (the Men's Union having rejected common colours); women's colours were later altered to maroon and light blue. Colours were awarded to individuals who had excelled in a particular sport.
By the inter-war period, the position of the Unions was much stronger, with sports embedded into University life. From 1914, the (men's) Union employed paid administrators, and funding was provided from a capitation fee levied on students. Membership of teams was limited to University members (previously many clubs adopted an 'open' policy). In 1921, the University introduced lecture-free Wednesday afternoons to encourage participation in sport. The standard of facilities gradually improved, with new grounds at Wythenshawe, south Manchester, and the McDougall Centre, with a pool and gym, which opened on campus in 1940.
In the post-war period, the number of member sports clubs increased rapidly, particularly for women's sports. In the 1960s, further extensions were made to the Firs, and a new boathouse was opened in Sale, Cheshire. Increasingly there was pressure for the Men's and Women's unions to merge, in part because some clubs did not wish to be separated by gender. In 1978, the UMIST Athletic Union formally broke away from the Union, having previously enjoyed had standing representation of the governing bodies of the Men's and Women's Unions. In 1981, the Men's and Women's Unions finally united to form a single Union. On merger, several men's and women's clubs also united.
By the 1990s, the University itself was taking a much more direct interest in the University sports. In 1993 a Director of Sport was appointed, with responsibility for managing University sports facilities. The University increased practical support for sports by funding the Armitage Centre at the Firs, which opened in 1986, to provide indoor sports facilities (with the McDougall Centre being gradually wound down in the last years of the twentieth century). By 2000 almost fifty clubs were members of the Union.