While at the grammar school of his native town he heard Richard Baxter preach as lecturer (appointed 5 April 1641) the sermons later published as 'The Saint's Everlasting Rest' (1653).
The living is described as sequestered in William Rastrick's list as quoted by Samuel Palmer, but James Halsey, D.D., the deprived rector, had been dead twelve or thirteen years.
In the plague year of 1665 Doolittle and his pupils moved to Woodford Bridge, near Chigwell, close to Epping Forest, Vincent remaining behind.
Returning to London in 1666, Doolittle was one of the nonconformist ministers who, in defiance of the law, erected preaching-places when churches were lying in ruins after the Great Fire.
But when he transferred his congregation to a large and substantial building which he had erected in Mugwell (now Monkwell) Street, the authorities set the law in motion against him.
When Charles II (8 March 1673) broke the seal of his declaration of indulgence, thus invalidating the licences granted under it, Doolittle conducted his academy with great caution at Wimbledon.
These migrations destroyed his academy, where his pupils had included Matthew Henry, Samuel Bury, Thomas Emlyn, and Edmund Calamy.
The academy was at an end in 1687, when Doolittle lived at St. John's Court, Clerkenwell, and had Calamy a second time under his care for some months as a boarder.
The Toleration Act 1688 left Doolittle free to resume his services at Mugwell Street, preaching twice every Sunday and lecturing on Wednesdays.
They consist of sermons and devotional treatises, including: His last work published in his lifetime was: Posthumous was* Doolittle married in 1653, shortly after his ordination; his wife died in 1692.