Jermyn was born at Hanworth, Middlesex, the country seat of his grandfather, the courtier Sir William Killigrew. He presumably grew up on the fringes of the Court, where his family was also represented by his father, Sir Thomas, a gentleman of the privy chamber, his uncle Sir Robert Killigrew*, and his cousin by marriage, (Sir) Humphrey May*.
Despite being under age when the 1624 Parliament was summoned, Jermyn was returned at Bere Alston, on the interest of the 1st Lord Mountjoy. He was probably recommended by his kinsman May, who had formerly served Mountjoy’s father (Charles Blount†).
In 1626 Jermyn briefly took centre stage in the Commons. By now one of Charles I’s equerries, he naturally resisted the impeachment of the duke of Buckingham. On 27 Apr. he acted as teller for the noes in the vote on whether to charge the duke with administering dangerous remedies to James I during the latter’s final illness. The House opted to persist with this contentious claim, but when it was presented before the Lords on 8 May, Sir Dudley Digges’s clumsy rhetoric seemed to imply that Buckingham had deliberately hastened the old king’s death, possibly even with Charles’s agreement. Digges was promptly arrested, but the Commons responded to this flagrant breach of its privileges by requiring all Members to declare that Digges had not in fact uttered the offending words. This development placed Jermyn in a quandary, and on 16 May he offered only a qualified protestation, asserting that although he had not personally witnessed Digges making the alleged remarks, he had heard them ‘from such that he cannot but believe that [Digges] spoke them’. This ill-judged show of support for Buckingham immediately backfired, as Jermyn was forced to admit that he was expressing the confidential views of the king’s immediate circle. As Digges had just been released again, and Charles was desperately trying to save face, Jermyn’s timing could scarcely have been worse. On the following day he informed the House, ‘(the king having given him leave), that the words he heard from the king himself, who since has found that he was misinformed’. After this spectacular faux pas, Jermyn understandably held his tongue for the remainder of the session.
Jermyn’s marriage to a wealthy heiress in 1629 brought him almost £5,000, but his father’s chronic indebtedness obliged him to pursue additional sources of income during the following decade.
Jermyn sat for Bury St. Edmunds in the Long Parliament, but joined the king as the Civil War approached. Officially discharged from the Commons in September 1643, he attended the Oxford Parliament four months later.
