Walmsley started his career in the amateur leagues, playing for Dewsbury Celtic, and his university team, Leeds Met Carnegie.

[9] St Helens reached the 2014 Super League Grand Final, and Walmsley was selected to play from the interchange bench in their 14–6 victory over the Wigan Warriors at Old Trafford.

[10][11][12][13] Walmsley's form in 2015 saw him named to the Super League Dream Team for the first time as well as earning him a nomination for the Man of Steel award.

[16][17][18] Walmsley played in St Helens 8-4 2020 Super League Grand Final victory over Wigan at the Kingston Communications Stadium in Hull.

[24] In the 2023 Challenge Cup semi-final, Walmsley suffered a knee injury following a cannonball tackle from Leigh player John Asiata.

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