Born around 1564, he was the son of Helen Graham and her husband John Forbes of Ardmurdo House in the parish of Kinkell, Aberdeenshire near Inverurie.
He graduated with a Master of Arts degree in 1585 from the University of St Andrews, becoming minister of Fettercairn in the Mearns in 1588, using this position to take an active role in the church politics of the day.
As a result, on 22 November 1604, he became Bishop of Caithness, retaining control of Fettercairn, something which created animosity with the anti-episcopal section of the Church of Scotland.
He was alleged to have granted the consent of the Scottish church, dishonestly, to the absolution of the Catholic magnate, George Gordon, 1st Marquess of Huntly, carried out on the king's wishes by the Archbishop of Canterbury.
Forbes, however, attempted and failed to succeed George Gledstanes to the Archbishopric of St Andrews, a position he was beaten to by John Spottiswoode.