Mary Ann Sinclair was born in a raupo hut at Pirinoa, southern Wairarapa, New Zealand, on 29 January 1864. She was the youngest of seven children of Scottish settlers Katherine McCallum and her husband, Donald Sinclair. A two-storeyed home was subsequently built, and by 1870 her father had completed the purchase of his 424-acre Burnside farm. He bought more land at Morrisons Bush, near Greytown, 10 years later.
Mary Sinclair married Donald Sinclair Sutherland, a shepherd, at Greytown on 20 August 1895 and she and her husband settled at Burnside. Over the years the Sinclair and Sutherland families built up a very close, reciprocal relationship with their Maori neighbours. Three children were born to Mary and Donald, one of whom, John Apiata (Hapi), was delivered in 1901 by Rena Ihakara and named in honour of Apiata Hakiaha, the Ngati Rakairangi chief at nearby Kohinui. The Sutherland household also included Mary's parents, sister Ann, brother Angus, a nursemaid (Nellie Pinkerton), and various governesses.
In 1894 Mary Sutherland's brother John Sinclair, in partnership with Iraia Te Whaiti, purchased Whatarangi, a large coastal station on the eastern side of Palliser Bay. When John moved away from the area around 1897 Mary became his agent, then began to buy his share. The partnership, known as Sinclair and Te Whaiti, continued to add to its holdings, acquiring the Kaiwaru block in 1907. By 1909 Mary and Donald Sutherland had also begun the purchase of their farm, Waihora, which had been part of the property of Mary's paternal great-uncle Angus McMaster.
When Mary's father died in 1914 she inherited the 142-acre Moroa block at Morrisons Bush (Burnside was left to her sister Ann). Mary's partnership with Te Whaiti was dissolved on his death in 1918, and the lands distributed between her and his family. She retained the Whatarangi name and continued to farm the homestead block. That year Mary and Donald moved into a large new homestead at Waihora.
Mary Sutherland's land holdings were widely dispersed and the family moved about between Whatarangi, Mangatoetoe (near Cape Palliser), Morrisons Bush, Waihora and Kaiwaru. By the mid 1920s Donald Sutherland was in failing health and at times Mary lived in boarding houses while he was in hospital in Wellington. He died in December 1927, and when he was buried Hoani Te Whaiti, Iraia's brother, laid a cloak that had belonged to his late daughter, Kere, on the coffin. Afterwards, Mary Sutherland continued to divide her time between her various homes but eventually settled at Waihora with her son Donald (Dugald).
Mary Sutherland was a strong-willed and domineering woman who made many land purchases but left others to arrange the money and do the actual farming. Her daughter, Katie, negotiated financial and legal affairs on Mary's behalf and from the mid 1930s Clifford Plimmer was important as her adviser. However, she treated the many people who worked for her with kindness and generosity and was called Aunt Mary by many of the local young Maori. Her life and that of her sister Ann were closely intertwined but at times their relationship was uneasy, and became more so when Mary's financial circumstances were shaky and Ann became her guarantor.
Ann Sinclair, who was of a much gentler nature, continued to farm Burnside. When Mary's son Hapi married in 1935, Ann leased the bulk of the farm to him. She moved across the road to the land where she and Mary had been born and built a small home for herself. She never had electricity installed, and as the years went by and her house became surrounded by gorse she became something of a recluse.
In 1945 Mary Sutherland bought a house in Featherston, where she lived with a housekeeper until her death there on 21 October 1948. She was buried at Featherston and her estate was divided between her three children. Unusually for a married woman of her time, Mary Sutherland had not deferred to her male relatives. Rather, she had been a driving force in land transactions and had maintained control over her own properties and financial affairs.