Arthur John Seamer was born in Tongala, Victoria, Australia, on 10 June 1878, the fourth child of William Seamer, a farmer, and his wife, Jane Matilda Townley. His father had worked on goldfields in California and Central Otago before settling in Australia. Arthur was just two when his father died, and the boy's childhood and teenage years were marked by the family's struggle to stay together and survive.
How much, if any, formal education Seamer received is unknown, but he apparently gained the opportunity to study when he was sent to stay with an aunt and uncle in Melbourne. His uncle was a Methodist local preacher, and his example influenced Arthur in his choice of profession. About 1896 the Salvation Army launched an appeal for missionaries to serve in Borneo or Java. Seamer applied, but because of his age was sent instead to New Zealand, in 1897. After training at Gisborne he worked among Maori in Taranaki and in the Urewera and Taupo districts, where he learned to speak Maori fluently. In 1899 his superiors were transferred to Melbourne and replaced by an Irishman who could not speak Maori and knew little about New Zealand. The consequent lack of progress in the Maori mission frustrated Seamer and other missionaries and they responded favourably to overtures from the Methodists. Seamer later said, 'I resigned and applied one week, and took up my appointment the next!' In 1901 he formally commenced service as a home missionary in Whangaroa.
In 1903 Seamer agreed to become a candidate for the ordained ministry. He served as a probationary minister at Springston in North Canterbury and in Rotorua before joining the Auckland Maori mission in 1905; its circuit comprised North Auckland, Auckland, Waikato and the King Country. He was ordained in 1907, and returned to the Whangaroa–Kaeo mission station. On 6 May 1907 he married Ida Marion Nisbet at Auckland; they were to have three children, one of whom died in infancy. His service with the Maori mission was interrupted by appointments to Petone (1909–11) and, for two years, Dunedin. He also served two terms as president of the New Zealand Young Men's Methodist Bible Class Union.
Seamer enlisted for military service in August 1915. A gastric ulcer caused him to return to New Zealand, but he re-enlisted, as a chaplain, in August 1916. In May 1918 he sustained an inner-ear injury from an exploding shell. It was to cause him constant pain and physical discomfort for the remainder of his life; head noises and sudden blackouts were regular. He was also diagnosed as suffering from shell-shock and neurasthenia, and after a period in hospital he returned to New Zealand in August.
Although not discharged from the army until May 1919, Seamer served as a minister in Addington in 1918. The following year, the Methodist conference returned him to Auckland and the Maori mission. In November he was admitted to Queen Mary Military Hospital, Hanmer Springs, with a recurrence of neurasthenia. He was released in February 1920 and in April assumed the position of general superintendent of the Maori mission, to which he had been appointed the previous year. In September that year he was devastated by the death of his wife from tuberculosis, and was to feel the loss and loneliness keenly for the rest of his life. In 1924 he became general superintendent of both home and Maori missions, to which he was to give outstanding service during a difficult period.
The post-war period saw the rise in the Rangitikei district of the prophet and healer Tahupotiki Wiremu Ratana. Seamer adopted a 'carefully considered policy' of silence on theological differences so as not to alienate the movement from Christianity. Methodist agents worked quietly to maintain Christian orthodoxy and became apostles when the Ratana Church of New Zealand was registered in 1925. As the movement developed, Seamer became an important adviser to Ratana, who, as a Methodist, looked to the church for guidance. In 1924 Seamer opposed the formation of a breakaway church led by Otene Paora; he later helped the Ratana church to formulate its creed. Seamer maintained a close friendship with Ratana, and his commitment to young people and their education resulted in the establishment of a school at Ratana pa serviced by Methodist lay teachers and deaconesses. He is commemorated in the town's Seamer Street.
From the late 1920s economic recession brought declining support for Methodist mission work as well as demoralisation of Maori communities. Seamer responded by utilising the Waiata Maori Choir. Formed in the mid 1920s around a handful of people who simply enjoyed singing, it grew under his tuition and discipline to become a highly trained, devoted and professional young Maori group. During the 1930s the choir travelled throughout the country, helping to raise morale among Maori and funds for the mission. Under Seamer's direction the choir toured Australia in 1933, 1935 and 1937. In 1937–38 it visited Great Britain, the climax of the tour being a command performance at Buckingham Palace. It also made a recording of much of its repertoire.
In 1933 Seamer was inducted as president of the Methodist Church of New Zealand. Despite ill health and the extra pressure of this position, his year of presidency was characterised by strong leadership and statesmanship. Nevertheless, the combination of national church responsibilities and physical strains were taking their toll, and in 1939 Seamer retired from his position as general superintendent of home and Maori missions and moved to Drury.
Seamer's unusual tolerance for the beliefs of others earned him the trust of Maori, to whom he was known as Te Himoa, or Te Hiima. His influence among Maori was soon again in demand by the church. In 1943 he moved to Hamilton as acting superintendent of the Waikato Maori circuit. He worked quietly alongside the reintroduced Pai Marire rituals without provoking resistance.
Seamer gradually became a close adviser to Te Puea. In the 1920s he had attempted to unite all Maori religious and political movements in a lobby group that would use the Pakeha system for their own gain; this had foundered because of Te Puea's opposition to Ratana. Now Seamer persuaded Te Puea to assist with the effort to keep alcohol out of the King Country, and encouraged her to establish a health clinic at Turangawaewae. He also became involved in the education of King Koroki's daughter Piki (later Queen Te Atairangikaahu). After Te Puea had suffered what was to be a fatal heart attack in 1952, she asked for Seamer and his colleague, Sister Heeni Wharemaru, to be at her side.
A concern for young Maori moving into the city for work or education resulted in the establishment of Te Rahui hostels in Hamilton: one for young women in Bryce Street, and one for young men in London Street. The name was suggested by Te Puea. In 1949 Seamer was made a CMG in recognition of his services to Maori and the community in general. He continued to give advice to a steady flow of influential people – including politicians – who came to see him at the men's hostel, Te Rahui Tane. He was cared for there by loyal Methodist Maori mission staff, in particular Heeni Wharemaru. Seamer died at Hamilton on 17 September 1963, survived by his son and daughter.