George Leslie Mackay was born near Embro, Zorra Township, Canada West on March 22, 1844, the son of Scots immigrants. Mackay decided to become a missionary when a graduate student in Edinburgh after hearing Alexander Duff call for foreign evangelism.In 1871, Mackay was sent to Mainland China by the Canadian Presbyterian Church but the following year he founded the first Canadian overseas mission in Tamsui, Taiwan.
Mackay practiced lay dentistry and trained local clergy. He worked in north Taiwan, married a Taiwanese, and had three children. Although natives destroyed several of his churches, he established 60 mission stations in 20 years.
In 1881, he raised funds in Oxford County to help build Oxford College, Tamsui, which later became a university. He was an inspiration to the evangelical missions movement in Ontario. He was an outspoken opponent of the Canadian head tax on Chinese immigrants.
George Leslie Mackay died in Formosa (Taiwan) June 2, 1901 at age 57.