[2] Allen attended St. Joseph High School in Metuchen, NJ where he scored 118 goals throughout his four-year varsity career.

During his tenure at Georgetown, Allen was a standout player where he earned All-American honors three times and was named as a finalist for the Hermann Trophy in 2015.

[3][4] At the conclusion of his 91-game career with the Hoyas, Allen recorded 50 goals and 17 assists, making him the all-time leading scorer in school history.

[6] On December 21, 2015, Allen signed a Homegrown Contract with the New York Red Bulls, where he stated in his first interview with the club, "I've waited a long time for this, all my life, but I enjoyed my college career and a lot of my success came from my teammates and my senior class was a big part of my success so I want to thank them.

[13] On October 2, 2016, Long scored his sixteenth of the season to help New York Red Bulls II advance to the Eastern Conference Semifinals of the 2016 USL Playoffs in a 4–0 victory over Orlando City B.

[15] On October 23, Allen recorded a hat trick and provided an assist in leading New York to a 5–1 victory over Swope Park Rangers in the 2016 USL Cup Final.

[18] On 6 May 2017, Allen recorded his first two-goal game of the season for New York Red Bulls II, netting on two penalty kicks, in a 3–1 victory over Harrisburg City Islanders.