Hargreaves played in the Wigan Academy from 2002. He made his first team début from the subs' bench against Salford City Reds in July 2004 and his full début against in August of that year. Hargreaves was given a 2-year full-time contract with the Wigan Warriors in August 2004. Hargreaves capped the 2004 season by being awarded the Under-21s Billie Joe Edwards and Craig Johnson MemorialPlayer of the Year trophy. Hargreaves was selected to play for Lancashire in the 2004 Academy Origin Series and in the England Under 18s squad that toured Australia in 2004 but was requested to stay at home to aid Wigan's injury crisis. He really impressed in the first team in 2005 when injuries to more experienced players provided an opportunity for the 20-year-old to establish himself within the first team squad. Hargreaves is notably the only not to conceded a penalty under Ian Millward's tenure. In July 2006 Bryn moved to Leigh Centurions for an initial months loan so that Wigan could complete the signing of Stuart Fielden.Commenting on the loan Wigan chairman Maurice Lindsay said, "Bryn is a fine player and I'm sure his time will come in the future. He is not in our first team at the moment although he did a fine job earlier in the year. We have allowed him to go to Leigh to get further experience and we wish him good luck." Bryn Hargreaves stayed on loan until the season's close and did not return to Wigan but instead was released from his contract with Wigan Warriors in 2006 as a part of Brian Noble's squad rebuilding for the 2007 season. At this time many clubs such as Leigh Centurions, Hull KR, Salford City Reds and Harlequins RL all expressed an interest in signing the promising young front rower. In November 2006 St. Helens, the then Super League and Challenge Cup champions, signed the ex-Wigan prop Bryn Hargreaves on a two-year deal. On Hargreaves, coach Daniel Anderson stated: “Bryn will be out fifth specialist prop next year and we are confident that we can develop him physically and technically over the next couple of years. “He was possibly thrown in at the deep end at too young an age as a starting prop at Wigan Warriors but we believe that he has genuine potential as a rotational squad member who can be patiently and methodically developed.” As 2006 Super League champions, St Helens faced 2006 NRL Premiers the Brisbane Broncos in the 2007 World Club Challenge. Hargreaves played from the interchange bench in Saints' 18–14 victory. He was then picked for Wales World Cup Qualifier against Scotland for whom he qualifies through his Grand Fathers Nationality. Bryn scored his first tries for St Helens on 13 June 2008, where he scored a hat-trick. He played in the 2008 Super League Grand Final defeat by the Leeds Rhinos. Following injuries to a number of Saints front-rowers in the 2008 season, Hargreaves established himself as a first-team regular alongside James Graham. In 2009 he has been the first-choice starting prop alongside Graham, ahead of Jason Cayless and Maurie Fa'asavalu. He played in the 2009 Super League Grand Final defeat by the Leeds Rhinos at Old Trafford.
On 3 September 2010, it was announced that Hargreaves had signed a 2-year contract with the Bradford Bulls for the 2011 & 2012 seasons. 2011 – 2011 Season Hargreaves appeared in two of the four pre-season games. He played against Halifax and Wakefield Trinity Wildcats. Bryn featured in fifteen consecutive games from Round 1 to Round 15. An injury kept him out for a few games but Hargreaves then played in nine consecutive games from Round 19 to Round 27. He also appeared in the Challenge Cup games against Halifax and Wigan Warriors. 2012 – 2012 Season Hargreaves featured in 18 consecutive games from Round 1 to Round 18. Hargreaves was injured for Round 19. Bryn also played in Round 20 to Round 27. He also featured in the Challenge Cup against Doncaster and Warrington Wolves. Hargreaves scored against Salford City Reds. Hargreaves announced his retirement in September 2012, "bitterly disappointed, angry and above all disillusioned by events off the field". Hargreaves said that he had been offered a new job that could sustain him for life, unlike a career in Rugby League, adding "We’re pieces of meat at the end of the day and we’re not treated with much respect in rugby league as it is."
