Beginning life with a paternal legacy of £100,1New Hist. Northumb. ix. 163. Delaval entered the navy, serving in the Mediterranean under Matthew Aylmer and Sir George Rooke. He combined his naval duties with diplomatic missions to the Emperor of Morocco, concluding a treaty for the redemption of English captives in 17002Charnock, Biog. Navalis, iii. 96. and an agreement not to molest each other’s ships in 1708.3Byng Pprs. (Navy Recs. Soc.), ii. 270-3. In 1710 he went for more than three years on a mission to Portugal.4HMC 13th Rep. VI, 191-3. Returned as a Whig for West Looe in 1715, he voted with the Government in every recorded division. In 1715, having amassed a sizeable fortune, he purchased Seaton Delaval from Sir John Delaval, the head of the elder branch of the family, asking Sir John Vanbrugh to draw up plans for a house. He wrote to his brother, Edward, 3 Apr. 1718:
I would be glad ... in my old age in repairing the old house, making a garden and planting forest trees, for which we may expect prayers when we are no more, praises I should call it for fear of being thought Popish;
adding that, although he would prefer to retire now, he would go to sea in the hope of getting the rank of admiral and then
go to my retreat with honour and advantage, for I esteem ... half-pay with liberty more than any office that must be attended of twice the value.5J. Robinson, Delaval Pprs. 11, 121-2.
He went to the Mediterranean under Sir George Byng, distinguishing himself at the battle of Cape Passaro, and was made a rear-admiral. He did not go to sea again, but was made a vice-admiral in a general promotion in February 1723, dying 22 June next of a fall from his horse.