In 2017 Allen moved up to the Joe Gibbs Racing Monster Energy NASCAR Cup Series No. 18 team with crew chief Adam Stevens and 2015 NASCAR Sprint Cup Series Champion Kyle Busch where from 2017 to 2020 the team went on to win 13 poles and 19 wins including the NASCAR Championship in 2019. In 2021 Allen moved to the Joe Gibbs Racing No. 20 NASCAR Cup Series team as the lead race engineer for crew chief Adam Stevens and driver, Christopher Bell.
