The Cardiff-born centre began his career with Dinas Powys at full back as an 11 year-old, later joining Glamorgan Wanderers where he played for two seasons in the Under 18 side. Allen made his Cardiff Blues debut against Newcastle Falcons in the LV=Cup in October 2011 at the age of eighteen.
Starting out as full back for Wales Under-18s, Allen moved to the centre whilst playing for the Wales Under-20s. He was in the Welsh side that made history by becoming the first Welsh team to defeat New Zealand (9-6) at the Junior World Championship in South Africa in 2012 where he was awarded the most improved player of the tournament by Under-20s head coach Danny Wilson.
Also featuring in the national sevens set-up, Allen made his debut for Wales in the second round of competition on the IRB Sevens circuit in Dubai in 2012 as a replacement against New Zealand. Allen then went on to be voted IRB try of the tournament in 2013, for his opening touchdown, after an overwhelming 49% of the votes.
Allen enjoyed a stunning 2013/14 season as he cemented his place in the Cardiff Blues line-up and was handed his first Wales cap during the autumn internationals of 2013. Allen's form throughout the season, despite a spell out with a shoulder injury, was rewarded when he signed a new long-term deal at the region and was named in the Wales squad to tour South Africa in June 2014.