A versatile player, Lomax is primarily a stand-off but is equally comfortable at both scrum-half and fullback, and featured more regularly on the wing in the early stages of his professional career.

[citation needed] Lomax made his first-team début for St. Helens in 2009's Super League XIV game against Wakefield Trinity in 2009, where he helped them to an easy 42–18 win.

Lomax's opportunities in the first team continued in 2010, with his first game of the season coming in a 38–6 win at Bradford Bulls in round 2 of 2010's Super League XV, where he featured on the right wing in place of the injured Ade Gardner.

Lomax enjoyed a breakthrough 2011 season, forming a makeshift halfback partnership with fellow youngster Lee Gaskell that saw St Helens reach another Grand Final, however they were unsuccessful, once again beaten by the Leeds Rhinos at Old Trafford.

7 jersey in 2012 as a result and enjoyed another good season, scoring 14 tries as the club's first-choice scrum-half, however Saints' run of successive Grand Final appearances came to an end as they were beaten by Warrington in the qualifying semi-final.

In 2013, Lomax was moved to the fullback position by coach Nathan Brown, and played 19 times, scoring 12 tries and kicking 25 goals as St Helens once again narrowly missed out on a Grand Final appearance.

In 2014 Lomax played in the first 13 matches for St Helens before being ruled out for the remainder of the season after suffering a severe knee injury in a 42–0 defeat against Catalans Dragons.

[30] His continued absence, along with the retirement through injury of Lance Hohaia and Paul Wellens, forced St Helens to bring in several short-term replacements, including Shannon McDonnell and Adam Quinlan.

[38] In round 18 of the 2022 Super League season, Lomax scored the winning try for St Helens with only one minute remaining as the club defeated Wigan 20-18 at Magic Weekend.

[39] On 24 September 2022, Lomax played in St Helens 2022 Super League Grand Final victory over Leeds and was awarded the Harry Sunderland Trophy as man of the match.

[42] In round 9 of the 2024 Super League season, Lomax kicked the winning drop goal for St Helens as they won 13-12 over Huddersfield with less than a minute remaining.