After becoming a minister, he was admitted as a sizar to Magdalene College, Cambridge. After graduating, he established a school in Shackelford, Surrey. Around 1802, he sold the school and accepted the position of morning preacher at the Foundling Hospital in London.
He was appointed rector of Hilgay, Downham, Norfolk in 1819 and served as professor of belles-lettres at the Royal Institution of Great Britain.
He is buried in the catacombs of the Foundling Chapel. Hewlett published on classical art
His renowned Vindication of the Parian Chronicle (1789), discussed the Arundel marbles. After the death of George Gregory, he continued publishing a newly edited Bible serially.
He published numerous books of sermons and theology as well as an Introduction to Reading and Spelling (1816).
His most important work, however, was his edition of the Bible (1812), which included five volumes of commentaries (1816).