He made his professional debut at the age of 16 years old with the club's reserve team, Philadelphia Union II, on July 18, 2020, against the Pittsburgh Riverhounds.
In the lead up to the CONCACAF Championship he scored a hat trick as a substitute in a come from behind win over River Plate's reserve team and added another goal in a 2–2 tie versus the Argentina U20's.
Sullivan comes from a soccer family: his mother is of Bangladeshi origin and his father, Brendan, played for five clubs over a six-year professional career in the A-League.
[11] His two other brothers, the younger twins Declan and Ronan, are also youth soccer players, with both currently attending YSC Academy and playing for MLS NEXT affiliate FC DELCO.
[12] His cousin is Chris Albright, an American international who played two seasons for the Union, and his grandfather is Larry Sullivan, who served as the head coach at Villanova from 1991 to 2007.