In 1827 he was ordained Church of Scotland minister of Roslin, near Edinburgh, and in 1828 he was translated to charge of North Leith.

In 1845 he was appointed professor of apologetics in the New College (Free church), Edinburgh, and in 1847, on the death of Dr. Thomas Chalmers, he was transferred to the chair of systematic theology, continuing there till his resignation in 1868.

In 1840, Buchanan was called to second charge of St Giles Cathedral in Edinburgh, in place of George Husband Baird.

From May 1845, he was additionally appointed as Professor of Apologetics at the newly-built New College of the Free Church on the Mound in central Edinburgh.

He is buried in the north-east section of the Grange Cemetery, facing the main north path.