| Honorific-Prefix: | The Right Honourable |
| The Earl of Portsmouth | |
| Birth Name: | Isaac Newton Fellowes |
| Birth Date: | 11 January 1825 |
| Parents: | Newton Fellowes, 4th Earl of Portsmouth Lady Catharine Fortescue |
| Tenure: | 9 January 1854 – 4 October 1891 |
| Predecessor: | Newton Fellowes, 4th Earl of Portsmouth |
| Successor: | Newton Wallop, 6th Earl of Portsmouth |
| Residence: | Farleigh Wallop Eggesford |
| Nationality: | British |
| Education: | Rugby School |
| Alma Mater: | Trinity College, Cambridge |
Isaac Newton Wallop, 5th Earl of Portsmouth DL JP[1] (11 January 1825 – 4 October 1891) was a British Peer and the son of Newton Fellowes, 4th Earl of Portsmouth and Lady Catharine Fortescue.[2]
Portsmouth was born as Isaac Newton Fellowes, but later resumed the family surname and arms of Wallop without Royal Licence when he succeeded to the peerage in 1854.[3] [4] He was the son of Newton Fellowes, 4th Earl of Portsmouth and Lady Catharine Fortescue, daughter of Hugh Fortescue, 1st Earl Fortescue.
He was educated at Rugby School and matriculated at Trinity College, Cambridge,[5] where he received a Master of Arts degree.
In 1872 Lord Portsmouth donated to his alma mater, Trinity College, Cambridge, a vast collection of papers by Sir Isaac Newton which had descended through Newton's great-niece Catherine Conduitt, daughter of John Conduitt and Catherine Barton, into the Wallop family by her marriage to John Wallop, Viscount Lymington.[6] [7]
A committee chaired by John Couch Adams and Sir George Stokes was appointed by the University to review the papers. Adams and Stokes selected only Newton's scientific papers, not wanting to blemish his reputation as an enlightened intellectual and scientist. After spending sixteen years cataloging Newton's papers, Cambridge University kept a small number and returned the rest to the Earl of Portsmouth.[8]
On 15 February 1855, Lord Portsmouth married Lady Eveline Alicia Juliana Herbert (21 December 1834 – 1 October 1906), daughter of Henry John George Herbert, 3rd Earl of Carnarvon, by his wife, Henrietta Anna Howard, daughter of Lord Henry Thomas Howard-Molyneux-Howard (yr. brother of Bernard Howard, 12th Duke of Norfolk).[9] [10] They had twelve children:
Lord Portsmouth declined the elevation to a Marquessate and the offer to become a Knight of the Garter from Prime Minister William Ewart Gladstone, thinking them 'beyond his merits'.[11]
He died on 4 October 1891 aged 66 and was succeeded in the Earldom by his son, Newton Wallop, 6th Earl of Portsmouth.[12]