He came through the London Broncos academy, but only played for the first team when it was named Harlequins RL in the Super League.
LMS was contracted to play for St. Helens until 2020, which would be his testimonial season. In close to twelve years, he won four League Leader's Shields in 2014, 2018, 2019 and 2022 plus five Super League championships. He also won the 2021 Challenge Cup with St Helens.
McCarthy-Scarsbrook was a product of the Quins RL Junior Academy squad. He toured Australia with British Amateur Rugby League Association U18s in 2004. He spent the 2005 season on loan at Hull FC's Senior Academy, and played a prominent role in Hull's reserve grade Grand Final winning season.[10]
McCarthy-Scarsbrook announced on 1 September 2010[15] that he was to leave Quins at the end of the season after turning down a new contract.
St Helens
On 3 September 2010, St Helens confirmed the capture of Louie McCarthy-Scarsbrook on a four-year deal starting in 2011, where he'll join up with St. Helens' other new signings Josh Perry and Michael Shenton. Upon Signing the deal Louie said, '"St Helens are entering an exciting time in their history and to be a part of that was a great pull."
He continued: "Not only will they have a new stadium in 2012 but the quality of the players already in the squad means they will be challenging on all fronts.
"I am looking forward to linking up with my new teammates and continuing to learn under Royce Simmons.
"I would like to thank Quins for giving me the chance to play rugby league and I have enjoyed my time there. The fans and players have been excellent with me and I wish them all well in the future."[16]
He played for St. Helens in their 2021 Challenge Cup Final victory over Castleford.[28]
On 9 October 2021, he played for St. Helens in their 2021 Super League Grand Final victory over Catalans Dragons.[29]
In round 23 of the 2022 Super League season, McCarthy-Scarsbrook scored a rare double in St Helens 60-6 victory over Hull F.C.[30]
On 24 September 2022, McCarthy-Scarsbrook played off the interchange bench in St Helens 24-12 victory over Leeds in the 2022 Super League Grand Final.[31]
On 18 February 2023, McCarthy-Scarsbrook played in St Helens 13-12 upset victory over Penrith in the 2023 World Club Challenge.[32]
On 20 September 2023, McCarthy-Scarsbrook announced he would be retiring from rugby league.[33]
He played 26 games for St Helens in the 2023 Super League season as the club finished third on the table. He played in St Helens narrow loss against the Catalans Dragons in the semi-final which stopped them reaching a fifth successive grand final. This would also be McCarthy-Scarsbrook's final game as a player.[34]
International career
McCarthy-Scarsbrook made his England début in the victory over Wales on 10 October 2008.[35] His second game for England was also against Wales, a 48-12 victory in the 2009 Four Nations tournament in which he scored a try.[7]