On March 7, 2020, a day after signing with Union II, McGlynn made his competitive debut for the side in the USL Championship against Loudoun United.[4] He came on as an 82nd-minute substitute for Luis Flores as Union II drew 0–0.[4] On August 5, McGlynn scored his first professional goal against New York Red Bulls II. His first-half stoppage time goal was the equalizer in a 3–2 victory for the Union II.[5] McGlynn finished the season for Philadelphia Union II with five goals in fourteen matches.[6]
McGlynn made his international debut for the United States at the under-16 level, playing against the Netherlands on November 14, 2019.[9] Jack is also eligible to represent Ireland due to dual citizenship through his Irish born father, and he has been approached by the Football Association of Ireland to do so if he chooses.[10]
On January 5, 2024, McGlynn received his first call-up to the senior national team for January training in Orlando ahead of a friendly against Slovenia.[14] On January 20, McGlynn came on as a substitute in the 61st minute in their 0–1 loss to Slovenia.[15]
Personal life
McGlynn is the younger brother of Conor McGlynn, who is also a professional soccer player for USL Championship club Rhode Island FC. On July 25, 2020, both Jack and Conor played against each other for Philadelphia Union II and Hartford Athletic respectively in the USL Championship.[16] They played against each other again on September 23, in which Jack scored the equalizer for Union II in a 1–1 draw.[17]