William is a centre forward who's come through the Manchester United Academy. His twin brother, Michael Keane, also plays for the club. A natural goalscorer, William stood out in the Academy, making his Under-18 debut at the age of just 14. He was a key member of the 2010/11 FA Youth Cup winning squad. He made the step up the Reserve side look easy and was rewarded with a first debut in December 2011, coming on as a substitute in a forgettable home defeat against Blackburn Rovers.
His career took an unfortunate turn for the worse when he suffered knee ligament injury on international duty in May 2011, just as it seemed like he was set for inclusion in the Manchester United first team on a more permanent basis. The injury would force him to miss the entire 2012/13 season.
William started the 2013/14 season back in training but his return from injury would come 16 months after the incident in a game against Bolton Wanderers; typically, he scored in it. Despite getting some goals for the Under-21 side, William unsurprisingly didn't resemble the pre-injury player he was. He looked slower, less instinctive and just not as confident. To help him regain some sharpness he joined Wigan on loan in November 2013 but struggled to make an impression and returned to Manchester United a month later with an injury. In January 2014 he joined Queens Park Rangers for the remainder of the season but wasn't a regular in the side.
With 7 goals in 9 games for the Manchester United U21 side, William joined Sheffield Wednesday on loan in January 2015 for the rest of the season.
On the 8th July 2015, William signed a loan deal with Preston North End, keeping him at the club for the entirety of the 2015/16 season. However Manchester United recalled William early in January 2016. Although he was to be involved with the first team, he used the Under-21 side for match fitness and scored a hat-trick away to Reading on the 11th January 2016 and five goals against Norwich City on 8th February 2016.
Four years after his debut for the Manchester United First Team, William made his second appearance for the club away to Sunderland as a substitute in February 2016. He followed that up with a substitute appearance away to Shrewsbury Town in the FA Cup but injured his groin in the game and missed the remainder of the season.
Late in August 2016, William signed a three year contract with Hull City.