His father descended from a family settled at Wye; his mother was Letitia, daughter of John Boys of Betshanger, Sandwich, where Brett was born.
He was educated at Wye grammar school, under John Paris and Samuel Pratt (later dean of Rochester).
The vicar, Mr. Gery, encouraged him to exchange his early Whiggism for Tory and High Church principles.
On the death of his father, his mother persuaded him to return (May 1696) to Spring Grove, where he undertook the cure of Great Chart.
He published a sermon 'on the remission of sins', in 1711, which gave offence by its view of sacerdotal absolution, and was attacked by Dr Robert Cannon in Convocation (22 February 1712).
He was presented at the assizes for keeping a conventicle, and in 1718 and 1729 complaints were made against him to Archbishop William Wake for interfering with the duties of the parish clergyman.
He took part in a negotiation which they opened in 1716 with the Greek archbishop of Thebais, then in London, and which continued till 1725, when it was allowed to drop.
Brett's account, with copies of a proposed concordat, and letters to the Tsar of Moscovy and his ministers, is given by Thomas Lathbury from the manuscripts of Alexander Jolly.
Brett supported Collier in proposing to reinstate the four usages that had been included in the first liturgy of Edward VI.