During the French Revolution he was sentenced to be executed, but escaped and eventually fled Europe to Demerara in South America[3] with his brother Joseph in 1793, then to New York in 1795.
[2] Darby made an impression on Grellet and under her and William Savery's influence he decided to join the Quakers (Society of Friends).
[2] Grellet became involved in extensive missionary work in prisons and hospitals across North America and most of the countries of Europe, and was granted meetings with many rulers and dignitaries, including Pope Pius VII, Tsar Alexander I, and the Kings of Spain and Prussia.
[7] The family home, the Isaac Collins House, in Burlington, New Jersey, is now listed on the National Register of Historic Places.
Grellet died in Burlington on 16 November 1855 and his body was buried there, behind the Quaker Meeting House at 340 High Street.