2013 USL Pro and Major League Soccer announce a multi-year agreement, beginning that season, to integrate MLS Reserve League play with USL Pro teams, first through team affiliations and "interleague" play, eventually fully merging MLS Reserves into the USL Pro structure.
Founded in 1985 by Francisco Marcos, the Southwest Indoor Soccer League was first intended as a minor indoor league associated with Major Indoor Soccer League. The league began with five teams, all owned by individuals who owned or operated indoor soccer arenas in the Southwest United States. League headquarters was in Austin, Texas. In 1986, Marcos' team, the Austin Sockadillos, entered the league giving it six teams. By 1989, the league had set its sights on greater prospects than remaining a small, semi-professional indoor league. A 1989 press release stated, "It is envisioned the league will be part of the USSF's plan to professionalize soccer in the USA prior to the 1994 World Cup, and the league plans to push its teams to be considered for the "three-tiered first, second and third divisions" plan the USSF envisions for soccer."
The USL began operations in 1986 as the Southwest Indoor Soccer League, a semi-professional indoor league. The league quickly expanded and added an outdoor season in 1989. In 1991, the league renamed itself the United States Interregional Soccer League. By then, the outdoor league overshadowed the indoor league which continued to slowly dwindle in importance. By the 1997–98 season, only five teams remained indoors while the outdoor season had over a hundred teams divided into a three-division structure. This led the league to stop running an indoor league.
In 2010, the USL began to consider relaunching an indoor season. However, it decided instead to take over operations of the Major Indoor Soccer League; beginning indoor operations in 2011. The champions were determined by a single games in 1987, 1988, 1992, 1993, 1994, 1995, 1996 and 1998. The champions were determined by a best of three series in 1998 and a best of five series in 1989, 1990 and 1991 and a home-and-home series in 2012 and 2013.
In 1989, the Southwest Indoor Soccer League added a summer, outdoor season known as the Southwest Outdoor Soccer League. In 1990, the league dropped both "indoor" and "outdoor" from its name as it ran both a semi-professional indoor and outdoor season. By 1995, the outdoor season had grown to such a size that the USISL, as it was known at the time, split the league into two levels, the aptly named, fully professional Professional League and the semi-professional Premier League. In 1996, the USISL added a third, higher, Select League. This was formed from the strongest teams from both the Professional and Premier League. The Select League, along with the competing A-League, both received official Division II status from FIFA. However, the A-League was in decline while the USISL was expanding. Therefore, in 1997, the A-League ceased operations and merged into the USISL Select League which was renamed the USISL A-League. From that point, the USL's three-level structure remained stable until 2010. Some teams left to start the North American Soccer League and the First and Second Divisions were merged to become the USL Professional Division.