Alexander Howard MacKay was born on May 19, 1848, in Plainfield, Pictou County, Nova Scotia.

He was an educator and scientist. He attended the Pictou Academy (1865) and graduated from Dalhousie University in Halifax (B.A., 1873; B.Sc., 1880). In 1873, he started as principal of the Annapolis Academy, but soon after he returned to the Pictou Academy to begin a 16-year career as a principal. There he strengthened the science program, organized the Pictou Academy Scientific Association, and pursued his own botanical and zoological research. In 1889, he became principal of the Halifax Academy. He was active in the Provincial Educational Association of Nova Scotia, founded the Summer School of Science for teachers in 1887, and served as Nova Scotia editor of the Educational Review (Saint John) (1887-1891). In 1891, MacKay began a 35-year term as superintendent of education for Nova Scotia, constantly urging the provincial Council of Public Instruction and local school trustees to reform the curriculum and to hire trained teachers. Throughout his life, he continued his scientific studies, his work on the flora and fauna of his own province, and developed a herbarium of native plants. From about 1908 until his death he edited the Proceedings and Transactions of the Nova Scotian Institute of Science, of which he was president from 1899 to 1902. He was president of the Dominion Educational Association (1895-1898) and in 1909, he became vice-president of the Simplified Spelling Board of New York. He served on the board of governors of Dalhousie (1888-1927) and he represented the university on the board of the Marine Biological Stations of Canada (1898-1926). He was awarded an honorary doctorate by Dalhousie in 1892 and by St. Francis Xavier College, Antigonish, in 1905. He was a director or board member of the Presbyterian College, the Halifax Ladies’ College, and the Victoria School of Art and Design, where he served as president from 1908 to 1924. Active in the North British Society, of which he was president in 1894, he served as vice-president of the Nova Scotia Historical Society from 1896 to 1902 and as president of the Halifax Canadian Club in 1912–1913.

In 1880, he married Maude Augusta Johnston (1851–1936). He died on May 19, 1929, in Dartmouth, Halifax, Nova Scotia.