Lomax kicked three goals in week one of the finals series as St. George Illawarra defeated Brisbane 48–18 in an upset victory at Suncorp Stadium.

[citation needed] Altogether Lomax played 2018 NRL season 4 games for St. George Illawarra winning two and losing two.

In the 2019 season Lomax played 10 games, scoring five tries and 15 goals before injuring his thumb at training, he returned in the round 19 loss against South Sydney.

In round 1123 of the 2020 NRL season, Lomax scored two tries and kick four goals in a 28–24 loss against rivals Cronulla-Sutherland at Kogarah Oval.

[12] In round 8 against the Wests Tigers, Lomax suffered a suspected broken thumb in the club's 16–8 loss at WIN Stadium.

[15] In round 6 of the National Rugby League, Lomax was involved in an incident during a try celebration where he decided to jump on the back of former teammate Tyson Frizell.

[17] In round 24, Lomax kicked a penalty goal in the final minute to win the game for St. George Illawarra 24-22 over the Wests Tigers.

[citation needed] On 1 July, news broke out by reporter Phil Rothfield that if Lomax formally requests a release from St. George Illawarra that they would allow him to speak to other clubs.

[citation needed] Later that day incoming St. George Illawarra coach Shane Flanagan blasted back on 2GB with him stating it was fake news and Lomax will stay as a Saints player like Ben Hunt.

[23] On 2 April, it was announced that Lomax would depart St. George Illawarra at the end of the season after being granted an early release from his contract which was due to expire in 2026.

[28] In round 14, Lomax scored a club record 32 points (three tries and ten goals) in St. George Illawarra's 56-14 victory over the Wests Tigers.

[29] In game two of the 2024 State of Origin series, Lomax scored two tries and kicked five goals in New South Wales 38-18 victory over Queensland.

[32] Zac Lomax made his test debut on the wing for Australia in the opening game of the Pacific Championships on 18 October at the Suncorp Stadium in Brisbane, kicking 3/4 goals in the Kangaroos 18–0 win over Tonga.