In his early life, he played his rugby at Trojans R.F.C in Hampshire. Then, in 1999 Allen toured South Africa with the South England School Boys side, Beating Natal 20–22. In the middle of the tour Allen was hospitalised with food poisoning, missing the end of the Tour. In 2000 Allen Won the Hampshire Cup with the Trojans Under 13 side, whilst attending Boundary Oak School at Roche Court, just outside Fareham. From there he moved on to Clayesmore School at the age of 13, where he was a member of Devine House. He was a member of the Clayesmore 1st XVs from year 11, effectively playing two years above his age-group.
In 2005, Allen joined Gloucester straight from Millfield School. At the start of the season, Allen helped Gloucester to triumph in the Middlesex 7s with some fantastic tries. He made his first team debut in Gloucester's Anglo Welsh Cup match against Bristol, which Gloucester won 34–6. Allen scored his first try for Gloucester in their 106–3 demolition of Bucuresti. His first Guinness Premiership try came 2 weeks later against Leicester at Welford Road. A glorious try against Worcester Warriors helped Gloucester to the European Challenge Cup Final. During his first season with Gloucester, Allen summed up his tremendous talent with a glorious brace of tries against London Wasps. He also played an important role in the defeat of London Irish in the European Challenge Cup Final.
After a great start to the season England came calling for Allen. His performance against Agen at Kingsholm, where he scored two tries in defeat, was a catalyst for his call up. On 28 November 2006, Allen signed a 2-year extension to his contract. He came back from national duty and continued his great form for Gloucester with 3 tries in 3 games in December. Allen finished the season with 9 tries including one in Gloucester's semi-final victory against Saracens. He was also voted the young player of the season by Gloucester's fans. Unfortunately for Allen and Gloucester they suffered defeat at the hands of Leicester in the Guinness Premiership final.
Allen started the season fantastically with 2 tries in the opening 2 weeks against Leeds Carnegie and Saracens. Tries against Ospreys and Bourgoin in the Heineken Cup helped Gloucester to the quarter-finals. Allen was one of the key players for Gloucester starting 29 games, more than anybody else in the squad. As well as Allen's obvious talent for breaking tackles and scoring tries his immense tackling plays a vital role to the team.
Allen, in his 4th season playing for Gloucester, firmly established himself as first choice inside centre alongside Mike Tindall at outside centre. The arrival of Olly Barkley from Bath presented a higher level of competition for Allen than in previous seasons. On the opening day of the season, Allen suffered an injury against Leicester which kept him out for 2 months. He returned in November against Saracens and his try in this game helped Gloucester to a win at Vicarage Road. Gloucester ended the season disappointingly and Allen moved to Leicester for the 2009/10 season.
In his first season at Leicester, Allen was faced with breaking into a first-choice centre pairing of Aaron Mauger and Dan Hipkiss. However, his ability to cover both positions and a series of injuries to his rivals allowed him to quickly establish himself in the side. Before long, he was a first choice centre and he marked his debut season at the club with the first major trophy of his career as Leicester won the Guinness Premiership final against Saracens. He finished the season with 6 tries in 28 games in all competitions.
His second season saw Allen as Leicester's first-choice inside centre alongside the teenage Manu Tuilagi. Allen put in a series of solid performances – both in attack and defence – that earned no international recognition, but saw him named as the Player's Player of the Season and scorer of Try of the Season for his solo score against Bath.
He made his England debut against New Zealand on 5 November 2006, partnering Newcastle Falcons' Jamie Noon in the centre. Allen received multiple stitches in his mouth halfway through that first half after being hit in the face by his New Zealand opposite number, but Allen was dropped after they lost 25–18 to Argentina.
He was an integral part of the England Saxons sides that won the Churchill cup in Canada / USA 2008 and again in 2010.