Philadelphia announced on Thursday that 14-year-old Sullivan has signed the largest homegrown contract in Major League Soccer history.
Sullivan, who joined Philadelphia's academy at the age of 10, now becomes the club's youngest ever player to be added to the first team roster as well as the fifth youngest player to sign a first-team contract in MLS history.
Although the financial details have not been disclosed, Sullivan's record deal runs until 2028 and it has widely been reported that Philadelphia have formed a partnership with Pep Guardiola's Man City, allowing the youngster to make a move to England in four years' time.
According to ESPN, Philadelphia were not authorised to discuss any aspects about Sullivan's built-in transfer to Man City, who have build a reputation for boating one of the best academies in world football.
However, the teenage midfielder has now confirmed the Citizens' involvement in his deal and is excited for what the future holds.
Sullivan: 'Man City agreement is the best plan'
Sullivan believes he is making the right decision by starting his professional career at Philadelphia, where he can develop in his home country before testing himself abroad at a higher level.
"I've always wanted to start my career here because this is my home and I've always been on the sidelines of Quinn's games (his older brother) and I've been in and around the facility when my uncle worked here," Sullivan told ESPN. "So, I've always been inspired and to play in front of this culture, and these fans are special.
"But I also think the collaboration between the Union and the City Group was - I think that did it for me. I always watch Man City.
"They're like every kid's dream team. For [Philadelphia and Man City] to come together and agree on something - I sat with my family and my agents and we decided that it was the best plan."
Philadelphia currently sit eighth in the Western Conference standings after 10 games and if Sullivan makes his senior debut before July 29, he surpass former Union prodigy Freddy Adu as the youngest player to appear in MLS.
"Special talent" Sullivan a rising star in the States
"I think it would be cool to obviously have my first record, but it doesn't really matter to me if I beat it or not," Sullivan said when asked about Adu's record. "I mean, everyone's on their own journey. It's not really where you start, it's about where you finish."
Philadelphia coach Jim Curtin has labelled Sullivan "a special talent, top talent not only in this country but in the world," and when asked if the youngster could make his debut in the coming weeks, he said: "If he's healthy, he'll do it."
Sullivan, who will wear the No.6 for Philadelphia, has enjoyed a rapid rise in his youthful career, making a name for himself with the club's Under-15s, Under-17s, the Next Pro team and with the USA youth national teams.
The midfielder was just 13 when he helped Philadelphia Under-15s reach the finals of the 2023 Generation Adidas Cup, beating the likes of Valencia, Real Madrid and Arsenal and scoring just a minute into their match against the latter.
Sullivan led the USA to victory in the 2023 CONCACAF Boy's Under-15 Championship, the nation's first-ever triumph at the tournament, and scored four goals to win the Golden Ball award as the best player of the tournament.
In 2024, Sullivan has featured twice for Philadelphia's second team, registering an assist for a winning goal in a game against New England Revolution II.