He was entered at Kendal Academy in 1742, under Caleb Rotheram, and went on to Glasgow University, where he matriculated in 1744, and was a favourite pupil of Francis Hutcheson and William Leechman.
On completing his studies he succeeded Charles Owen, D.D., as minister of Cairo Street Chapel, Warrington, Lancashire, where he was ordained on 8 December 1747.
Soon after his settlement the Percival family left the established church and attached themselves to Seddon, thought to be a liberal divine of Arian views.
At the same time he succeeded Joseph Priestley in the chair of belles lettres; his manuscript lectures on the philosophy of language and on oratory, in four quarto volumes, were preserved in the library of Manchester College, Oxford.
A subsequent meeting at Preston on 10 September 1751 declared in favour of ‘a proper variety of public devotional offices.’ Next year the ‘provincial assembly’ appointed a committee on the subject; a long controversy followed.
On 16 October 1760 a number of persons in Liverpool, headed by Thomas Bentley, agreed to build a chapel for nonconformist liturgical worship, and invited several dissenting ministers to prepare a prayer-book.