He was the second son of John Henry Forbes, Lord Medwyn, a judge of the court of session, and grandson of Sir William Forbes of Pitsligo. He studied first at the Edinburgh Academy, then for two years under the Rev. Thomas Dale. the poet, in Kent, passed one session at Glasgow University in 1831 and, having chosen the career of the Indian civil service, completed his studies with distinction at Haileybury College. In 1836 he went to Madras and secured early promotion, but in consequence of ill-health he was obliged to return to England. He then entered Brasenose College, Oxford, where in 1841 he, obtained the Boden Sanskrit scholarship, and graduated in 1844.
He was at Oxford during the early years of the movement known as Tractarianism, and was powerfully influenced by association with Newman, Pusey and Keble. This led him to resign his Indian appointment. In 1844 he was ordained deacon and priest in the Church of England, and held curacies at Aston, Rowant and St Thomas's, Oxford; but being naturally attracted to the Episcopal Church of his native land, then recovering from long depression, he removed in 1846 to Stonehaven, the chief town of Kincardineshire. The same year, however, he was appointed to the vicarage of St Saviours, Leeds, a church founded to preach and illustrate Tractarian principles.
On October 28, 1847 Forbes was consecrated to succeed Bishop Moir in the see of Brechin; He removed the episcopal residence to Dundee, where he resided till his death, combining the pastoral charge of the congregation with the duties of the see. When he came to Dundee the churchmen were accustomed owing to their small numbers to worship in a room over a bank. Through his energy several churches were built, and among them the pro-cathedral of St Paul's.
He was prosecuted in the church courts for heresy, the accusation being founded on his primary charge, delivered and published in 1857, fit which he set forth his views on the Eucharist. He made a powerful defence of the charge, and was acquitted with a censure and an admonition. Keble wrote in his defence, and was present at his trial at Edinburgh. Forbes was a good scholar, a scientific theologian and a devoted worker, and was much beloved. He died at Dundee on the 8th of October 1875.
- A Short Explanation of the Nicene Creed (1852)
- An Explanation of the Thirty-nine Articles (2 vols, 1867 and 1868)
- Commentary on the Seven Penitential Psalms (1847)
- Commentary on the Canticles (1853)
See Mackey's Bishop Forbes, a Memoir.