Conduitt was the son of Leonard and Sarah Conduitt, and was baptised at St Paul's, Covent Garden, London, on 8 March 1688. He was admitted to St Peter's College, Westminster School, as a King's scholar in June 1701. In 1705, while at Westminster, he was elected a Queen's scholar to Trinity College, Cambridge, with three others He was admitted there in June of that year and matriculated to the University, but did not graduate, staying only two years.
By 1707, based on his own account he was "travelling" in Holland and Germany. In September 1710, he became judge advocate with the British forces in Portugal. He was a "very pretty gentleman" according to James Brydges. From October 1710, he acted as the Earl of Portmore's secretary when the latter arrived in Portugal (N&Q). During this time, he kept the Earl of Dartmouth informed as to the Portuguese court. He returned to London by October 1711 with Lord Portmore. During the following year, he was made a captain in a regiment of the dragoons serving in Portugal, but by September 1713 he had been appointed Deputy Paymaster General to the British forces in Gibraltar. The posts appear to have been remunerative, and in May 1717 he returned home to England a richer man.
In June 1721, Conduitt was elected, on petition, a Whig member for Whitchurch, Hampshire, which he represented during the 1720s as a loyal supporter of Walpole's government. He took an active interest in the running of Isaac Newton's office of Master of the Mint in the latter years of Newton's life, and he was appointed in his stead in March 1727 after Newton's death. He attempted to collect materials for a life of Newton, but after starting, he quickly stopped. In 1728, he was somewhat unhelpful to John Newton, the heir to Isaac Newton's real estate, and Newton had to resort to the Chancery courts to get satisfaction.
By the early 1730s, Conduitt had become a relatively prominent parliamentary speaker, defending the government on a number of issues, including Walpole's maintenance of the Septennial Act. In 1734, he was re-elected to his seat, but chose to represent Southampton. On 12 January 1736, he introduced a successful bill repealing an early 17th-century act against conjuration and witchcraft.
Shortly after his arrival back in England, he became acquainted with Sir Isaac Newton and his niece Catherine Barton. After what must have been a whirlwind romance, they applied to the Faculty Office for a licence, which was granted on 23 August 1717, to marry at St Paul's, Covent Garden. Catherine, then aged 38 years, described herself as 32 years old, Conduitt more correctly as about 30. Despite the licence, they instead married three days later on 26 August in her uncle's parish in the Russell Court Chapel in the church of St Martin in the Fields. Perhaps in an effort to dignify himself for his impending marriage to one of London's famous daughters, Conduitt obtained for himself a grant of arms from the College of Heralds on 16 August.
The couple had one daughter, named after her mother, born 23 May 1721 and baptised in the same parish of St Martin's on 8 June. Partly as a result of his antiquarian interests, Conduitt was elected a Fellow of the Royal Society on 1 December 1718, proposed by the president, and his uncle by marriage, Sir Isaac Newton.
In 1720, Conduitt acquired the estate and house at Cranbury Park, near Winchester; towards the end of his life, Sir Isaac Newton took up residence at Cranbury with his niece and her husband until his death in 1727.
Conduitt died on 23 May 1737 and was buried in Westminster Abbey on 29 May to the right of Sir Isaac Newton. His wife Catherine died in 1739 and was buried with him. In his will dated 1732, he left his estate to his wife and made her guardian of their underage daughter Catherine. On his death, the trustees sold the estate at Cranbury Park as well as estates at Weston and Netley, near Southampton to Thomas Lee Dummer, who succeeded him as MP for Southampton
His daughter Catherine later married John Wallop, Viscount Lymington (died 1749) in 1740. He was the eldest son of John Wallop, 1st Earl of Portsmouth, and their son, John Wallop, succeeded as the second earl of Portsmouth.
Sir Isaac Newton
| Master of the Mint
|Parliament of Great Britain|
| Member of Parliament for Whitchurch
With: George Carpenter 1721–1722
Thomas Vernon 1722–1727
Thomas Farrington 172
John Selwyn 1727–1734
John Selwyn, Jr 1734–1735
John Selwyn, Jr
| Member of Parliament for Southampton
With: Sir William Heathcote
Thomas Lee Dummer