He was the eldest son of Michel Guerdain, Constable of Trinity, and Marie, daughter of Aaron Stocall, Constable of St Saviour.
In 1627, being then a BA, he was admitted to Jesus College, Cambridge, and as a member of that college took his MA. He became Fellow of Queens in 1631, but had vacated that post by October 1633. He graduated Doctor of Medicine at Rheims In April 1634. On 7 November 1640 he was cited before the College of Physicians and admonished, because he had been practising in London without a licence for four and a half years.
When the Civil War began, it became known in Jersey that he had joined the Parliament's side, so Sir George Carteret seized the large house at Trinity, known as La Guerdainerie, which Guerdain had inherited from his father, and gave it to Col Smyth to establish a mint in it, where silver plate of the Royalists could be transformed into half-crowns for the King's use.
Furnaces and crucibles were installed, but then it was discovered that Smyth was using them to flood the island with base coins for his own profit, and he had to flee to escape the hangman's rope.
In 1649 Sir George Carteret cut down all the trees on Guerdain's estate to build a store in Elizabeth Castle.
By this time Guerdain was sufficiently well-known in England as a Parliamentarian for his name to be borrowed by the author of an anti-Cromwellian satire: ”A most learned, conscientious, and devout Exercise or Sermon held forth the last Lord's Day of April at Sir P T's house in Lincoln's Inn Fields by Lieutenant General O Cromwell, as it was faithfully taken in Characters by Aaron Guerdain.
Master of the Mint
In the same year, by a strange coincidence, he whose own house had been transformed into Royalist Mint, was appointed Master of the Mint in the Tower of London at a salary of £400 in place of Sir Robert Harley, who had refused to take the King's head off the coins. He held this post until 1669, and was responsible as 'Master-worker of the Monies’, for the coining of all the new Commonwealth money.
In 1652 a State Paper asserts that his post "should be worth £1,500, now that he is Melter also". He was put on the Commission of Peace that he may better prevent disorders in the Mint", and in 1653 it is mentioned that part of his duty was to hunt down and prosecute all false coiners.
He still practised as a physician. In 1653 Parliament ordered "the two Dr Guerdains" (the other was his brother Denis) "to examine sick soldiers to select those who should be sent to Bath".
In 1655, when Cromwell nominated eleven new Jurats for Jersey, he was one of those selected, but he declined to serve. At the Restoration he lost his post as Master of the Mint, but seems to have made his peace with the authorities, for in December 1664 he and his brother were both elected honorary fellows of the College of Physicians. He died in 1676.