Sims played for his local NRL club, St George Illawarra Dragons, from 2003 to 2007, making his NRL début against the Penrith Panthers in round nine of the 2003 season. In 2004 he suffered a suspected broken jaw, having been elbowed in a wayward shoulder charge by Newcastle's Clint Newton. Newton was sent off and suspended for twelve weeks. Sims however managed to play out the game. During the 2007 season, the Dragons told Sims he was no longer wanted at the club, and he signed for the Brisbane Broncos on a three-year contract starting in 2008. He played 81 NRL matches for the Dragons over five seasons. Ashton Sims was one of the club youngest debutants in the history of the club..
Brisbane Broncos
Sims made his Brisbane Broncos début in the Round 1, 2008 clash against reigning wooden spooners, the Penrith Panthers, in a 48–12 win. Later that year, an elimination semi-final against the Melbourne Storm, Sims started the match in the back-row. The Broncos were leading at the time, but went on to lose the game in the final minute, after Sims dropped the ball with two minutes remaining in the match, which led to the match winning try by Greg Inglis. Sims described it as "one of the low points" of his career. Sims played almost 60 games for the club and was one of the club's highest paid players, he played in the back-row and in the front row during his time at the Broncos. Shortly after the Broncos' semi-final exit, Sims was named in the Fiji squad for the 2008 Rugby League World Cup and was later confirmed for the final squad. He had only visited Fiji once in his life—for a holiday—but qualified to play for the country because his mother was born there. He was Fiji's starting second-rower for all four of their matches; Fiji qualified for the tournament's semi-final for the first time. Sims fell out of the Broncos' starting line-up early in the 2009 season, finding himself playing for the Norths Devils in the Queensland Cup. He was recalled to the Broncos' first thirteen for the team's finals campaign, in which they fell one match short of qualifying for the Grand Final. However, he only played nine NRL matches in the following season, and in August signed to play for the North Queensland Cowboys in 2011.
North Queensland Cowboys
Sims moved to North Queensland with his younger brother, Tariq, who had played in the Broncos' Under-20s team. He played in 24 matches for the Cowboys in the 2011 season, and was a substitute for their qualifying final loss to Manly. Ashton Sims led the way for NRL props in offloads in 2011, making 42 to his teammates. Sims hit career best form for the Cowboys in 2012 averaging 108 metres and 22 tackles, he did an admirable job in the Cowboys starting pack while rep star Matthew Scott was out with a foot injury. On 11 May 2012 it was announced that the oldest Sims brother had re-signed with the club for a further two seasons, which saw him at the club until the end of the 2014 NRL season.
Ashton represented Fiji in the 2013 Rugby League World Cup. He appeared in all 5 of Fiji's matches. In May 2014, Ashton captained Fiji in the 2014 Pacific Rugby League International. Fiji failed to win the test match and in doing so, failed to qualify for the 2014 Four Nations. In October 2017, Ashton represented Fiji once again in the World Cup, held in his home country Australia. There, Fiji beat New Zealand in the Quarter Finals but unfortunately suffered a loss in the semifinals to Australia and cousin - Reagan Campbell-Gillard.
