William Turnbull Leach was born on March 1, 1805, in Berwick-upon-Tweed, England.

He was a clergyman and educator. In 1827, he graduated from the University of Edinburgh (M.A.) and after studying theology he was licensed as a minister of the Church of Scotland. He moved to Canada in the early 1830s and accepted a call to be a minister of St. Andrew’s Church in Toronto in 1835. In 1842, he resigned from St. Andrew’s, turned to the Church of England, and in 1843, he became the first rector of St. George's Church in Montreal. In 1846, Leach established a connection with McGill College which lasted until his death. Over the course of 40 years, he was professor of classical literature and lecturer in mathematics and natural philosophy (1846–1853); fellow and vice-principal (1846–1886); lecturer in logic, rhetoric, and moral philosophy (1853–1872); dean of the Faculty of Arts (1853–1886); and Molson professor of English language and literature (1872–1883). Due to all these duties, he resigned from his position at St. George's Church in 1862.

In 1835, he married Janet “Jessie” Carnegie Skirving (1805–1848); in 1850, he remarried Eliza Margaret Easton (1809–1866), and in 1869, he remarried Louisa Gwilt (1849–1935). He died on October 13, 1886, in Montreal, Quebec.