 He was born in New Glasgow, Nova Scotia, the son of physician Alexander Forrest, an immigrant from Scotland, and Barbara Ross McKenzie, and was educated at the Free Church College in Halifax, and graduated from the Presbyterian College at Truro in 1865, then received the Doctor of Divinity degree from Queen's University, Kingston, in 1873.He was ordained in 1866 and became the pastor for St. John's Church in Halifax.Forrest married Annie Prescott Duff in 1871.