Sir Donald Macmaster was born on September 3, 1846, in Williamstown, Glengarry, Ontario.

He was a Canadian lawyer and a politician in both Canada and the United Kingdom. He studied law at McGill University, was called to the Quebec bar in 1871, and set up practice in Montreal. He served as Crown Prosecutor for many cases. He was called to the Ontario bar and appointed Queen's Counsel in 1882. He represented Glengarry in the Legislative Assembly of Ontario from 1879 to 1882 and in the House of Commons of Canada as a Conservative member from 1883 to 1887, when he lost his seat. In 1905, Macmaster emigrated to the United Kingdom and settled in London. In 1910, he was elected to the House of Commons as a Conservative member for the Chertsey division of Surrey, holding the seat until his death. He was created a baronet in 1921.

In 1880, he married Janet Macdonald (1860–1883) and in 1891, he remarried Ella Virginia DeFord (1864–1923). He died on March 3, 1922, in Chelsey, London, England.