By 2008 Harlequins coach Brian McDermott was tipping McCarthy-Scarsbrook for a place in England's World Cup squad.

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."

St Helens reached the 2014 Super League Grand Final, and McCarthy-Scarsbrook was selected to play at second-row in their 14-6 victory over the Wigan Warriors at Old Trafford.

[24][25][26] He played in St Helens 2020 Super League Grand Final victory over Wigan at the Kingston Communications Stadium in Hull.

[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.

He played in St Helens narrow loss against the Catalans Dragons in the semi-final which stopped them reaching a fifth successive grand final.