Charnley was born in Chorley and joined Wigan from local amateurs Wigan St Patricks. He made numerous appearances for the Under 20s in 2010 before joining Blackpool on dual registration and then Hull Kingston Rovers on loan. He was a player around the fringes of a first team opportunity at Wigan before the possibility came to join Hull Kingston Rovers for a month. Injuries meant he was given the chance to play first team rugby by Justin Morgan making his Super League début. Charnley showed great promise on his début for Hull KR against Leeds in 2010 and picked up the club man of the match award. He scored five tries in five appearances which included a hat-trick in Hull KR's home win over Castleford and the opening try in the derby clash with Hull.
Charnley made his first team début for Wigan at Craven Park against Hull KR in August where he scored a try off the bench. He played on the wing for Wigan in the 2011 Challenge Cup final, scoring the first try (and breaking a bone in his hand) in his side's victory over Leeds.
In 2012 Charnley played for the England Knights team and in Super League as he scored 31 tries to help Wigan to the League Leaders Shield. This put Charnley in 3rd place in the all time top try scorers in a regular season in the summer era as only Lesley Vainikolo (36 tries) and Danny McGuire (35 tries), both in 2004, have scored more tries in a Super League regular season. Charnley then went onto an even better 2013 season helping Wigan Warriors to the double ( challenge cup winners and super league champions) scoring a record breaking 43 tries ranking him 1st. Although the game ended in a 36-14 defeat for Wigan, Charnley scored two tries in the 2014 World Club Challenge against NRL champions Sydney Roosters. On 18 June 2014, he scored 5 tries in a 48-4 victory over Widnes Vikings.  Despite missing two months of action due to injury, Charnley still scored at a rate of over a try a game with 23 tries in 21 games in the 2014 season.