[2][3] On 3 July 1632 he married Ithamaria Reginald (also rendered as Ithamara or Ithumaria, with the surname Reginolles), sister of the writer and polymath Bathsua Makin.

Pell spent much of the 1630s working under Hartlib's influence, on topics in the area of pedagogy, encyclopedism and pansophy, combinatorics, and the legacy of Trithemius.

[6] As part of a joint lobbying effort with Hartlib to find himself support to continue as a researcher, he had his short Idea of Mathematics printed in October 1638.

[7] His reputation and the influence of Sir William Boswell, the English resident, with the States-General procured his election in 1644 to the chair of mathematics in Amsterdam, after an earlier attempt immediately after Martin van den Hove left for Leiden had failed.

For this he put in a large effort soliciting help and testimonials: from Bonaventura Cavalieri, his patron Sir Charles Cavendish, René Descartes, Thomas Hobbes, Mersenne, Claude Mydorge, and Gilles de Roberval.

From 1654 to 1658 Pell acted as Cromwell's political agent in Zürich to the Protestant cantons of Switzerland; he cooperated with Samuel Morland, the English resident at Geneva.

[14] Pell was described in Zürich by the English traveller Sir John Reresby in about 1656 as "a strange unknown person, not unsuiting the people he was sent to, nor the master [Cromwell] he came from.

However Pell's negotiations were long drawn out and he returned to England to deliver his report only shortly before Cromwell's death.

[19] This new edition by Pell of what was essentially Rahn's work included a great deal of additional material on number theory, amounting to a reply to the 1657 book Exercitationes mathematicae by Frans van Schooten.

[22] In 1673 Pell met Leibniz in London, and was able to inform him that some of his mathematical work had been anticipated by François Regnaud and Gabriel Mouton.

He lived, on the invitation of Dr Daniel Whistler, for a short time in 1682 at the College of Physicians, but died at the house of Mr Cothorne, reader of the church of St Giles-in-the Fields.

In 1654,[26] Thomas Pell signed a treaty with Chief Wampage and other Siwanoy Indian tribal members that granted him 50,000 acres (20,000 ha) of tribal land, including all or part of the Bronx and land to the west along Long Island Sound in what is now Westchester County, extending west to the Hutchinson River and north to Mamaroneck.

Having no children, he left his estate to his nephew Sir John Pell (1643–1702), one of the mathematician's four sons, who travelled from England to New York and took up residence there as the first Lord of the Manor of Pelham.