Mullen previously played for the Houston SaberCats, the San Diego Legion and the Newcastle Falcons.
[1] Mullen also played age-grade rugby for Munster before moving to the United States to attend Texas A&M University.
[6] In August 2018, Mullen signed a four-month contract to play for the Newcastle Falcons in England's Premiership Rugby.
After the 2019 Rugby World Cup in Japan, Mullen signed for the San Diego Legion for the 2020 season.
[10] Mullen is qualified to represent the United States at the international level on account of having an American grandparent.