Campbell was born on 25 December 1719 in Aberdeen (it is unlikely that there was much significance given to his birth date, since Scottish Calvinists, such as Campbell's father, Rev. Colin Campbell, despised both popish rituals and holidays). At the age of fifteen, Campbell attended Marischal College where he studied logic, metaphysics, pneumatology (philosophy of mind and/or spirit), ethics, and natural philosophy. After graduating with his M.A. in 1738, Campbell decided to study law and served as an apprentice to a writer to the Signet in Edinburgh. He began gravitating towards theology after attending lectures at the University of Edinburgh. After serving out his term as an apprentice, he returned to Aberdeen and enrolled at both King's and Marischal Colleges, University of Aberdeen as a student of divinity. Because of the tumultuous political landscape in Scotland (see Jacobite rising of 1745), Campbell's divinity examinations were delayed until 1746 when he received his licence to preach. Within two years, he received ordination at the parish of Banchory Ternan.