Mackay-Smith was born on June 2, 1850, in New Haven, Connecticut, the son of Nathan Smith and Grace Caroline Bradley.

He was educated at St Paul's School in Concord, New Hampshire, and then at Trinity College from where he graduated in 1872.

[1] He was ordained deacon by Bishop John Williams in 1876, after which he became assistant at All Saints' Church in Worcester, Massachusetts.

He was ordained priest on December 19, 1877, by Bishop Benjamin Henry Paddock after which he became missioner at Grace Church in South Boston.

Philip M. Rhinelander was elected and consecrated as a coadjutor that same year and Mackay-Smith announced his resignation to become effective on February 10, 1912.