[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.

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.

After spending sixteen years cataloging Newton's papers, Cambridge University kept a small number and returned the rest to the Earl of Portsmouth.

[7] 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).

[2][3] 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'.