In 2007 Ashton signed for Northampton Saints in rugby union's Premiership, before moving to Saracens in 2012, Toulon in 2017, Sale in 2018 and Harlequins in 2020; he finished his career at Leicester Tigers in 2023.

He made his debut for England in March 2010 and played regularly until 2014, including in the 2011 Rugby World Cup where he finished as the tournament's joint leading try-scorer.

Ashton was given the number 1 shirt, previously worn by Radlinski, at the start of 2007 despite the arrival of former Bradford Bulls full back Michael Withers.

Although some criticisms were made about his defensive abilities, he earned a call into the England squad at the end of 2006 while Great Britain were on tour in Australia and New Zealand.

[2] On 19 March 2007 it was announced that Ashton had signed to play rugby union for Northampton Saints, after Wigan granted him an early release from his contract.

[5] On 1 September 2007, Ashton made his debut for Northampton at Franklin's Gardens as a late substitute for Will Harries against London Welsh in National Division One.

In the final at Twickenham, Saracens played Bath, who had a strong set of backs, all of whom were England internationals, and were expected to be tough opponents.

However, Bath's challenge faltered as Saracens established early control of the game and did not lose the lead, eventually winning 28–16.

[12] Ashton's strong performances throughout the season meant he finished in joint-2nd place on the try scoring tables with 13 tries, the same as fellow Saracens winger Chris Wyles, behind Exeter Chiefs forward Thomas Waldrom.

In the 2015-16 season Ashton helped Saracens to both the Premiership title and the European Rugby Champions Cup, starting both finals.

[13][14] In the 2016-17 season Ashton helped Saracens retain their European Rugby Champions Cup title, scoring a try in the final against Clermont.

[16] Ashton played a single season in the Top 14, scoring 24 tries in 23 appearances to break the competition's try-scoring record previously held by Napolioni Nalaga.

[21] On 25 January 2021, Ashton was granted early release from Harlequins as he joins with Premiership rivals Worcester Warriors with immediate effect until the end of the 2021-22 season.

[23] Ashton joined Leicester in February 2022 and made his debut against former club Northampton Saints on 12 February 2022, he made a try saving tackle while on during Freddie Burns' first half Head Injury Assessment, but was then sin binned in the 70th minute after replacing Guy Porter just 3 minutes earlier.

He gave the scoring pass to fellow Saints player fullback Ben Foden, who was also making his first Six Nations start.

[33] Ashton represented England at the 2011 Rugby World Cup in New Zealand and finished as the tournament's joint leading try-scorer with six.

Again, Ashton's defensive capabilities were the subject of much discussion during this campaign and he did not play for England during the 2014 autumn internationals or 2015 Six Nations.

However, during Saracens' ultimately winning campaign in the 2014–15 premiership season, Ashton consistently performed at a high level and this promising form resulted in Stuart Lancaster recalling him back into the England camp, including him in his 50-man training squad for that year's 2015 World Cup, alongside fellow Saracens winger David Strettle, who had not played for England since 2008.

In November 2018, he made his first England appearance in four years, coming off the bench in their 12–11 win over South Africa, before being named in the starting fifteen against New Zealand.

NH = Tournament not held Up to dates as of 16 April 2023[44] Ashton's support play, which he learnt during his days as a rugby league footballer, has been praised and is often cited as the main reason that he is such a prolific try scorer.

[47] He has been praised and rebuked in equal measure for it, with critics concerned about the risk of dropping the ball in the act of scoring.