费城联II

Since the formalized partnership between United Soccer League and Major League Soccer in 2013, MLS teams had begun to form and field their own teams on the then third tier. This endeavor was largely driven to fill in the missing piece of player development between high school/college players and professional soccer players. In June 2015, the Philadelphia Union sent out a survey to fans gauging interest in fielding their own USL team. The survey input and directives from the Union front office eventually led to the creation of a team that would play in the Lehigh Valley, officially announced on August 19, 2015. This announcement made the Union the ninth MLS team to independently own and operate a USL club. The announcement also coincided with dissolving the previous affiliation with USL side Penn FC (formerly Harrisburg City Islanders), originally established in 2010. After surveying fans for a name of the team, the results ended overwhelmingly in favor of “Bethlehem Steel FC” honoring the previous incarnation of the team in the early twentieth century. Bethlehem Steel FC would become the “missing link” of player development between YSC Academy and first team for the Philadelphia Union. The club began to take shape hiring former Union assistant coach, Brendan Burke as the first head coach of the team's history on October 29, 2015. On December 3, 2015, Burke made his first signing to the team in Derrick Jones, who was a player developing in the Union Academy.

First seasons (2016–2018)

In preparation of their inaugural season, Steel FC built a roster with a combination of USL veterans (Mickey Daly, Ryan Richter, Bolu Akinyode), Philadelphia Union draft picks (Josh Yaro, Fabian Herbers), and key international players (James Chambers, Cory Burke). This team would be largely supported by Union players on short-term loans in addition to academy players from YSC Academy. The team's first season saw glimpses of positives but ultimately struggled down the stretch of the season, going winless in their last 12 matches.

On January 5, 2017, the United States Soccer Federation granted USL provisional Division II status; making the 2017 season the first time the Steel FC would compete as a Division II team. Steel built on their inaugural season continuing to integrate more amateur players from YSC Academy and other young players. The emergence of Seku Conneh and Cory Burke as reliable scoring threats helped Steel FC double their previous season's win total, finished 8th in the Eastern Conference and earned their first playoff appearance.

In 2018, Steel FC took another step forward with their best season to date, setting a record earning 50 points and 14 wins. Steel FC finished 6th in the Eastern Conference and won their first playoff match against in-state rivals Pittsburgh Riverhounds. Advancing to the Conference Semi-finals, Steel FC ultimately lost to eventual 2018 USL Champions, Louisville City FC.

Relocation and rebranding (2019–present)

At the conclusion of the 2018 season, Bethlehem Steel announced that it would play the 2019 season at Talen Energy Stadium in Chester, Pennsylvania; home of Steel's MLS parent club, Philadelphia Union. The relocation was prompted due to Goodman Stadium not having stadium lighting, limiting the scheduling possibilities and being in non-compliance with USL requirements. The team front office pledged to continue seeking out potential venue solutions that meet the USL Championship requirements for the 2020 season.

On December 12, 2019, the Philadelphia Union announced that the Bethlehem Steel would change their name to Philadelphia Union II for the 2020 USL Championship season.

On October 16, 2020, the Philadelphia Union announced that Union II would be withdrawn from the USL Championship and did not play competitively in 2021.

MLS Next Pro

The club announced on December 6, 2021, that it was joining the 21-team MLS Next Pro for the inaugural 2022 season.