Agnes Carmont was born at Castle Douglas, Kirkcudbrightshire, Scotland, probably on 2 September 1829, the daughter of Elizabeth Caven and her husband, John Carmont. She was raised largely in the household of her uncle, a Glasgow doctor, and by assisting him in his dispensary acquired a knowledge of medicine. In 1850 she was engaged for three years as companion help to Mary Ann Clifford, who with her husband, Charles Clifford, farmed the huge Flaxbourne estate in Marlborough, New Zealand. She arrived at Wellington on the Phoebe Dunbar in November 1850.
On 23 May 1854, at Wellington, Agnes Carmont married Hector McDonald, a trader. A native of Scotland, Hector McDonald was born probably in 1812, at Rothesay Bay, Isle of Bute. His parents' names are unknown, but his grandfather was Donald McColl, a Presbyterian minister, and maternal uncle of Donald McLean, the future native minister of New Zealand. At the age of six Hector emigrated with his family to Tasmania, Australia, where they settled at the River Derwent. At an early age he joined a whaling vessel, and in 1832 established a shore-whaling station at Kapiti Island, New Zealand. When the colonial settlement of Port Nicholson (Wellington) was established in 1840, he turned to trading. He ran two schooners between his store at Otaki and the new settlement, trading in Maori produce.
On his arrival in New Zealand Hector McDonald had met Te Rauparaha, and the two developed a mutual respect. At an unknown date he married Te Kopi, a niece of Te Rauparaha. Te Kopi is thought to have died giving birth to their only son, Hugh, who was born on 14 April 1848. He was raised by Hector and Agnes, who were to have five daughters and five sons of their own.
About the time of his marriage to Agnes Carmont, Hector McDonald acquired a lease of some 12,000 acres of coastal land between the Ohau River and Poroutawhao, on which he ran cattle and some 2,000 sheep. The land was leased from Muaupoko and several hapu of Ngati Raukawa, whose settlements were included in the run. 'Rent day', when the tribal leaders gathered at the homestead for their payment, was a major local festival, with races and wrestling matches held on the beach. In later years the McDonalds organised race meetings on a grassy flat near Lake Horowhenua. The family grew up bilingual and were educated solely by their mother. In 1858, when a coach service began along the coast between Wanganui and Wellington, Hector McDonald built an accommodation house and changing stables at the mouth of the Hokio Stream. Agnes and Hector ran the accommodation house for 11 years. More than a convenient stop for travellers, it provided a link between the developing colonial society and the coastal Maori villages of Horowhenua.
For many years Agnes McDonald used her medical experience to treat local Maori. Her work was of critical importance during the 1860s when epidemics of scrofula and influenza caused heavy mortality in the district. She is said to have found an effective treatment for scrofula, using iodine. In the 1870s she applied to Donald McLean and was authorised to make up a medicine chest at government expense and to replenish it on each visit to Wellington.
The district was not untouched by the conflicts of the 1860s. The Pai Marire movement attracted many of the younger Maori men. The issues of war were hotly debated at a meeting at Pukekaraka, near Otaki, in June 1865, at which Hector McDonald was present as a spectator. The meeting decided, however, that Manawatu should be a zone of peace. In 1869 the McDonalds leased the accommodation house and moved inland to a new homestead near the head of the Hokio Stream. It was also close to the boundary between two antagonistic tribal groups: Muaupoko and Ngati Raukawa. When skirmishing broke out along the boundary line, fighting occurred close to the homestead. On one occasion, when Hector was at Otaki conferring with the authorities, shots were exchanged over the roof of the house. Agnes sent her eldest son to demand that the fight move further off.
During these years Hector McDonald provided the government with information and advice. The homestead provided hospitality for the rare official visitors, especially during 1873 when Native Land Court judge John Rogan awarded greatly increased territory to Muaupoko. Hector McDonald grew increasingly anxious, as his health declined, about the security of his tenure. He made several applications to McLean for a Crown grant, suggesting that it be made in the name of the son of Te Kopi, Hugh. It appears that some grant was envisaged, but delays ensued. In the meantime the Native Department paid a supplement which doubled his salary of £10 as Horowhenua postmaster, a position he had held since June 1872.
In December 1876 Hector McDonald was elected to the first Manawatu County Council as member for Horowhenua Riding. However, he died on 7 August 1878, after collapsing in the street outside an Otaki hotel. He left an estate of only £47. The second son, Hector Hugh, succeeded to the farm and post office. Agnes McDonald took over the post office in 1883 and continued to run it until December 1894. That year she moved to Heatherlea, a substantial estate north of Levin purchased by her son, John Roderick. She attempted to take the Horowhenua post office with her, renaming it Heatherlea, but as it served few, if any, people, indignant questions in Parliament brought its closure within four months. She died at Opaki, near Masterton, on 28 November 1906.
By the end of the 1880s the Horowhenua region had been opened up to European settlement. To the new settlers the McDonalds were figures of romance and the acknowledged authorities on things Maori. In a period when the area was still predominantly Maori, they had adapted themselves to the dominant culture, and provided an important link between Maori society and the immigrant population.