In 1820 he entered the dissenting theological academy Manchester College, which was then at York, and which eventually became Harris Manchester College, Oxford.
He was ordained in 1825 as the minister of the Octagon Chapel, Norwich.
In 1828 he moved to the York Street chapel (now Browning Hall) in Walworth, south London.
In 1844 the congregation gave him a set of plate with an inscription by Charles Dickens, who at that time attended his services.
In August 1858 Tagart went on an official visit to the Unitarians of Transylvania.