[2] Allen is the youngest winner of a Grand Prix on the Longines Global Champions Tour, and was a team gold medalist at the 2017 European Championship in Gothenburg.

[citation needed] Allen finished third overall at the World Cup Jumping Finals in Las Vegas; both were on Molly Malone V.[5] He won the 2014 Dublin Grand Prix on Molly Malone V, and was a member of the Irish Nations Cup team at Dublin (on Romanov) which won the Aga Khan Trophy.

[10] He was on the gold medal-winning Irish team at the 2017 European Championships with Hector van d'Abdijhoeve, despite a fall during the competition.

[12] Allen purchased Harley vd Bisschop (a stallion formerly ridden by Nicola Philippaerts) in October of that year, selling Hector van d'Abdijhoeve ro Denis Lynch in January 2019.

[13][14] He and GK Caspar won the World Cup Qualifier at the Royal Agricultural Winter Fair in Toronto, and had several wins in 2020 before the season was halted due to the COVID-19 pandemic.