Byrnes was admitted as a barrister in Victoria on 8 July 1884 and returned for a Queensland admission on 5 August; he then began a successful career as a barrister. Byrnes' talent brought him to the attention of fellow barrister Sir Samuel Griffith, then Premier of Queensland, who had him appointed Solicitor-General with a seat in the Legislative Council. Byrnes stood down from the Legislative Council to successfully stand for Cairns in the Legislative Assembly in 1893. He represented Cairns until 1896, after which he represented Warwick in the Legislative Assembly from 1896 to his death in 1898.