Mellish was elected to the Legislative Assembly at the 2017 state election, defeating former Newman Government minister Tracy Davis with a 4.3% swing.[2] He was re-elected at the 2020 election, defeating LNP candidate and former Brisbane City Councillor Amanda Cooper with a 4% swing, increasing his margin to 5.4%. After his re-election, Mellish was promoted to the position of Assistant Minister to the Premier for Veterans' Affairs, Trade and COVID Economic Recovery.[3] Mellish assumed two additional ministries – Transport and Main Roads, and Digital Services – in December 2023. Mellish was re-elected for a third term in 2024, with a 0.04% margin of victory.