On his father's death, Seddon's education was undertaken by the congregation of Cross Street, Manchester.
He was at Stand Grammar School under William Walker; then at the Kendal Academy (entered 1733) under Caleb Rotheram;[1] and at Glasgow University from 1739, where he was a pupil of Francis Hutcheson.
[1] On leaving Glasgow, he became assistant at Cross Street to Joseph Mottershead, and was ordained on 22 October 1742.
A contemporary account describes the excitement produced by his utterances; his outspokenness won for him increased respect, though he made few converts.
His Discourses on the Person of Christ, Warrington, 1793, were edited with An Account of the Author, by Ralph Harrison, at the suggestion of Joshua Toulmin.