Thomas Bannatyne Gillies married Catherine Douglas on 1 June 1852 shortly before migrating to New Zealand. The couple settled, with other members of TB Gillies' family, in Otago where he later practised law. TB Gillies became involved in local and then national politics. In 1852, following Catherine's death in childbirth, TB Gillies with his four children moved to Auckland where he took up a legal practise. In 1867 he married for a second time to Agnes Sinclair, a niece of Andrew Sinclair, former colonial secretary. The couple had two children. Gillies re-entered politics, became superintendent of Auckland Province, and also served as an MP. In March 1875 he was appointed judge of the Supreme Court of New Zealand for the Auckland district, the first New Zealand judge to have qualified for admission to the Bar by New Zealand examination.