Alexander Miers MacGregor was interested in fossil hunting from a young age. He studied geology at Queens College, University of Cambridge, which awarded him the BA degree in 1911. The next year he was appointed as geologist and assistant curator at the Rhodesia Museum (later the National Museum of Zimbabwe) in Bulawayo. In July 1915 he left the museum to join the Geological Survey of Southern Rhodesia, directed by H.B. Maufe*. That same year he published his first paper on the geology of southern Africa, 'Notes on a graphic intergrowth of diopside and ilmenite from the Bembesi diamond field, South Rhodesia' (Transactions of the Geological Society of South Africa, 1915). The next year he produced a comprehensive paper on 'The Karoo rocks and later sediments north-west of Bulawayo' (Ibid, 1916). As World War I (1914-1918) had broken out by this time he enlisted in February 1916 and saw active military service in France. There he attained the rank of Lieutenant in the Artillery and was severely wounded.
Returning to his post in the Geological Survey of Southern Rhodesia in August 1919, MacGregor described the geology of various regions and mines along the gold belts of the territory during the next 20 years, in publications of the Geological Survey. Other noteworthy papers by him dealt with 'Note on older Palaeolithic implements from the Umguza and Bembesi valleys' (South African Journal of Science, 1920), 'The problem of the Pre-Cambrian atmosphere' (Ibid, 1927), 'Fossil reptilian remains from Glen Curragh farm, Nyamandhlovu district' (Proceedings of the Rhodesia Scientific Association, 1928-1929), and 'Geological notes on a circuit of the great Makarikari Salt Pan, Bechuanaland Protectorate' (Transactions of the Geological Society of South Africa, 1930).
MacGregor was director of the Geological Survey of Southern Rhodesia from 1946 to his retirement in 1948. In 1947 the University of South Africa awarded him the degree Doctor of Science (DSc) for a thesis, submitted to the Natal University College, consisting of his published work on the geology of Southern Rhodesia. After his retirement he was employed as geologist by various companies, from 1954 to 1956 by the Anglo American Corporation to start their geological office in Salisbury (now Harare).
MacGregor was a Fellow of the Geological Society of London and a member of the Mineralogical Society of London. He became a member of the Rhodesia Scientific Association in 1912 and served as president for 1946-1947. His presidential address dealt with 'The influence of life on the face of the earth'. In 1914 he became a member of the Geological Society of South Africa, served on its council from 1947 to 1955, and as president in 1950. In his presidential address he described 'Some milestones in the Precambrian of Southern Rhodesia'. He became a member of the South African Association for the Advancement of Science in or before 1919, served as joint secretary of Section B (which included geology) at the association's congress in Bulawayo in 1920, and as president of Section B in 1950. He was awarded the Draper Memorial Medal of the Geological Society of South Africa in 1947 and was honoured as an Officer of the Order of the British Empire (OBE) in 1949.