Henry Thomas Joynt Thacker was born at Okains Bay, Banks Peninsula, New Zealand, on 20 March 1870. His parents, Essy Joynt and her husband, John Edward Thacker, a former editor of the Sligo Guardian, had come to Canterbury in 1850. John Thacker became a dairy farmer and sawmiller at Okains Bay, and established Thacker Brothers, a profitable business dealing in wool and stock.
Having shown ability at Okains Bay School, Henry Thacker was sent to Christchurch Boys' High School from 1883 to 1886, and in 1887 he enrolled at Canterbury College. He was a notable footballer, representing Canterbury province in 1889 and 1891. In 1892 he completed a BA and travelled to Edinburgh to study medicine. While there he gained a rugby blue and after completing his MB and CM in 1896, he went to Dublin where he became an FRCSI in 1898. That year, on 16 March, he married Monica Alexandra Morrison in Dublin. There were no children of the marriage.
On his return to New Zealand in 1898 Thacker commenced private practice in Christchurch. He was a visiting physician at Christchurch Hospital, but resigned in 1902 after refusing to examine a patient diagnosed as having bubonic plague. He was widely respected as a general practitioner for treating all comers, regardless of their ability to pay. The only non-farming partner in Thacker Brothers, on his father's death in 1896 he had also become a large landowner with a comfortable private income. As one of Christchurch's early motoring enthusiasts he always had a new and expensive car, and employed a uniformed chauffeur to keep it in immaculate condition. He was once fined for overtaking at the 'dangerous speed' of 15 miles per hour.
Thacker was actively involved in community life. In 1904 he was elected president of the South Island Brass Bands' Association, retaining this post until 1928 and becoming their patron. He served as president of the Canterbury Automobile Association in 1905–6. A keen oarsman in his youth, Thacker became a rowing administrator, helping to train Richard Arnst, who became the world champion sculler in 1910. He also supported hockey and cricket, but his most important contribution to sports administration was as first president of the Canterbury Rugby Football League (1912–29). He donated the Thacker Shield and became a life member of the Canterbury League in 1920. In 1925 he was a trustee for the purchase of grounds in Woolston, which were named Monica Park after his wife.
Thacker also developed strong political interests. Having been briefly chairman of the Okains Bay Road Board, he served on the Lyttelton Harbour Board (1909–24) and was elected to the Christchurch City Council in 1910. He was chairman of the committee which inaugurated the Lake Coleridge electric power scheme, before losing his seat in 1912. He also contested the mayoralty in 1911 and 1912. Gaining a reputation for his witty and forceful speeches, he gave enthusiastic leadership to the campaign for a canal and port in Christchurch from 1908 to 1925. The canal scheme led him into national politics, and he stood for the Lyttelton seat as a Liberal in 1908 and for Christchurch East as an independent Liberal in 1911, losing the 1911 election only on the second ballot. In 1914 he was nominated by the Liberals for Christchurch East and won easily when T. H. Davey withdrew. Thacker held this seat until 1922.
At the outbreak of the First World War, Thacker won widespread popularity for demanding an inquiry into insanitary conditions at Trentham Military Camp, a move which he later claimed had saved many lives. In Parliament he soon made his mark as an able and independent debater, declaring that he would always 'speak straight out what I think'. His early proposals included the freezing of trout and deer for export, better ventilation and washing facilities in schools, and a state-funded accident insurance scheme.
During the 1918 influenza epidemic he visited Okains Bay and found nearly every household in the district stricken. Thacker set up an emergency hospital there and sent his chauffeur to collect the most serious cases from the other bays. In 10 days he treated over 140 cases and lost only three to pneumonia. His timely intervention undoubtedly averted a much higher death toll.
This humanitarian work may have tipped the balance in Thacker's favour in a close-run contest for the Christchurch mayoralty in April 1919, in which he stood as an independent on a mixed platform of civic progress and sports promotion. He is remembered as one of the city's most colourful mayors. In 1920 he was civic host during the visit of the prince of Wales and the governor-general, Lord Jellicoe. When the students' capping procession of that year made fun of him as mayor, he responded with equal good humour in a speech from his car, and treated all of the graduates to morning tea. He repeated this largesse every year until 1929.
During his term of office the city absorbed four neighbouring boroughs (Spreydon, Papanui, Woolston and Bromley), and Thacker constantly promoted the idea of greater Christchurch. He presided at the public meetings which fiercely debated proposals for a war memorial and finally resulted in the Bridge of Remembrance (opened in 1924). On 16 December 1922 Monica Thacker as mayoress laid the foundation stone for the new civic chambers. Although defeated for the mayoralty in 1923, Thacker was re-elected to the council in 1929 and served until 1931, during which time he was appointed deputy mayor and inaugurated a tree-planting scheme along the lower reaches of the Avon River. These years saw a revival of the canal proposal, and as chairman of the Port Christchurch League (1929–32) he led a deputation to the prime minister, George Forbes, in 1930; however, the canal was never built.
Henry Thacker was a big man (said to weigh 18 stone) who combined a commanding figure with a bluff outspoken manner: he once described the Canterbury Women's Institute as 'a gathering together of prudes' and 'a set of broody hens'. He always dressed smartly, and from his habit of wearing gloves and spats gained the reputation of being a dandy. His public meetings were unpredictable and entertaining, thanks to his ready wit and expert handling of hecklers.
Thacker was a lifelong advocate of exercise and physical fitness, endorsing the Sandow method and condemning corsets for women. Ironically, in later life his own movement was restricted by partial paralysis of his left leg, which made him use a walking stick. In numerous lectures and newspaper articles he preached the principles of a rational diet, with emphasis on wholemeal bread, vegetables and fruit. He always ate pineapple before a speech, 'to assist the voice'. In his last years he became well known throughout New Zealand for his radio talks on health and diet, which gained a large following in the 1930s.
Henry Thacker died at Christchurch on 3 May 1939 survived by his wife, Monica. For over 30 years one of the city's best-known public figures, Thacker was respected for his courage, persistence and strong individual views. Though disappointed in his long campaign for 'Port Christchurch', he had made significant contributions in local government and sports administration.