Montagu was one of the younger of the eight sons of the judge Sir Edward Montagu, of Boughton, Northamptonshire and Elizabeth Harrington. He was the grandson of another judge Sir Edward Montagu and his third wife Helen or Eleanor Roper. He matriculated at Christ's College, Cambridge in December 1588 and was admitted at Middle Temple on 11 May 1593.
In 1593, Montagu was elected Member of Parliament for Brackley. He was elected MP for Malmesbury in 1601 and for Wells in 1614. He became Master of Requests to King Charles I and was Knighted on 28 July 1616.
In November 1640, Montagu was elected MP for Huntingdonshire in the Long Parliament. Since he was reputed to be a man of great wealth, the Commons at the outset of the Civil War were infuriated by his refusal of their request to contribute £2000 to their cause. He was expelled from the Commons and committed to the Tower of London in 1642 as a known Royalist. He was released two weeks later after promising to contribute £1000 to the Parliamentary cause, although it seems that he only paid £200. He spent his remaining years in retirement.
Montagu lived at Hinchingbrooke House, Huntingdonshire, England. He married firstly in 1619 Paulina Pepys, daughter of John Pepys, of Cottenham, Cambridgeshire, England and sister of Richard Pepys and Thomas Pepys, grandfather of Samuel Pepys. It is often suggested that it was a love match, as Paulina had no fortune and was not her husband's social equal. Their only surviving son Edward was created Earl of Sandwich in 1660. They also had a daughter Elizabeth who married Sir Gilbert Pickering, 1st Baronet and had twelve children, including Elizabeth Creed.
Paulina died in 1638. In 1642 Sidney remarried Anne Isham, daughter of Gregory Isham of Northamptonshire, and widow of John Pay of Westminster. The marriage was very happy: in his will he praised her as a "religious, virtuous woman, as loving and contenting to me as my heart can desire", and left her generously provided for. She died in 1676.
Samuel Pepys, as he admitted, owed his start in life to the "chance without merit" which made his great-aunt Paulina the mother of the first Earl of Sandwich.
- "Montagu, Sidney (MNTG588S)". A Cambridge Alumni Database. University of Cambridge.
- Willis, Browne (1750). Notitia Parliamentaria, Part II: A Series or Lists of the Representatives in the several Parliaments held from the Reformation 1541, to the Restoration 1660 ... London. pp. onepage&q&f=, false 229–239.
- Knights of England
|Parliament of England|
|| Member of Parliament for Brackley
With: Richard Bowle
Sir Henry Knevett
| Member of Parliament for Malmesbury
With: Sir William Mounson
Sir Roger Dalyson
Sir Thomas Dalyson
| Member of Parliament for Wells
With: Thomas Southworth
Sir Capel Bedel
| Member of Parliament for Huntingdonshire
With: Valentine Walton