According to family information Margaret McKinlay was born at Newmains, Lanarkshire, Scotland, on 8 September 1844. She was the only daughter of James McKinlay, a colliery manager, and his first wife, Mary McNeil. Margaret worked as a dairy maid on neighbouring farms, but after a quarrel with her father's third wife, she decided to emigrate to New Zealand. Being outfitted for the journey by her two uncles and granted an assisted passage by the Canterbury provincial government, she paid only £8 10s. herself. She sailed from Gravesend in the Sebastopol on 17 January 1863.
On arrival at Lyttelton on 21 May 1863 Margaret McKinlay and the ship's third officer, George Gleigg Gardner, were virtually engaged, and he wanted to desert the ship. However, she persuaded him to see out the Sebastopol's return to England. Also aboard, as cabin passengers, were H. C. H. Knowles and his wife, of Glentui station, North Canterbury. Mrs Knowles engaged Margaret McKinlay to work at their homestead. She worked there until Mrs Knowles's death in the following year. She then worked for Mrs R. L. Higgins, wife of the manager of Murphy's run, Cust.
George Gardner returned to Lyttelton in March 1864. The couple were married in the new Wesleyan chapel, Christchurch, on 15 March 1866, and set up their first home in Salt's Gully, Lyttelton. Soon after, George received a family inheritance and in January 1868, influenced by Margaret, who was by nature a countrywoman, purchased a 40 acre section at Cust. By 1880 the Cust holding had increased to over 200 acres. Two years later, during the Canterbury wheat bonanza, George built a flour mill on the Cust River. On 5 February 1885, while handling a loaded gun, he fatally injured himself, leaving his widow with a family of nine children. A tenth child was born after George's death.
Margaret Gardner then took over the management of both farm and mill. The latter was a large business, supplying bakers in Christchurch, Rangiora and Oxford. She organised the purchase of grain and the selling of flour, and prepared her son Ralph Gardner to take over what was known as 'Mrs Gardner's mill'. A disastrous fire destroyed building and stock in March 1898, but a new mill, described as being 'one of the most complete in the colony', was built within a year. In 1899 Margaret Gardner sent Ralph to North America to study the use of steel rollers in flourmilling. In 1918 the family concern became the North Canterbury Co-operative Flourmilling Exporting and Agency Company, which in 1930 purchased the Zealandia Milling Company and took its name.
Margaret Gardner continued to supervise the farm, adding 180 acres in 1903. In 1904, at the age of 60, she sold the Cust land and bought 2,962 acres at Scargill for £20,000. She farmed this land, named Eastcott, with her three youngest sons until 1910, when the property was divided equally among them. Later she lived in Christchurch where she died on 19 June 1929.
From Scottish dairy maid to Canterbury landowner and businesswoman, Margaret Gardner made the most of the situation thrust on her by her husband's death. She was a large woman with a commanding presence and considerable physical strength. She also had an imperious will. Her married life was hardly unruffled: George Gardner possessed a quick temper and hers was a smouldering one; she also used to beat her sons, well into their teens. But a strong will and good health were essential if she was to handle successfully the heavy responsibilities thrust upon her.