He entered the Navy as a boy and when he was only 22 was given his first command, the Eighth Lyon's Whelp; in 1636 he commanded the Third Lyon's Whelp, and then the Expedition, in which he transported arms from the Tower of London to Edinburgh in 1640. He then commanded a small squadron in the English Channel, and in 1642 he escorted the Portuguese Ambassador to Lisbon in the Garland. On the outbreak of the Civil War he declared for Charles I, but his men mutinied and he was imprisoned. On his release he joined the King at Oxford and in 1644 went to the Continent to raise funds for the Royalist cause. He returned to England: he and his brother Walter were with Prince Rupert when he surrendered Bristol, then they went to Brussels to join their brother Arthur, who in 1658 was created first of the Slingsby baronets of Bifrons. Robert later returned to England alone, and in 1650, like so many defeated Royalists, he compounded i.e. paid a fine in return for being left with sufficient means to live on. According to his sister Dorothy Nightingale, he was then living with their widowed mother at York. Robert was then described as being "infirm and wounded, and not likely to live long".
At the Restoration he was given his father's old office of Comptroller of the Navy, and was created the first and last of the Slingsby baronets of Newcells. He had already presented the King with his book "The Past and Present State of His Majesty's Navy" which argued for regular payment of sailors' wages, prohibition of private trading in goods by Naval officers and the encouragement of merchant shipping. Samuel Pepys praised the great efforts Slingsby had taken over the book, but added drily that he had too high an opinion of his own work. Despite such occasional jibes, a warm friendship sprung up between Slingsby and Pepys: Slingsby invited Pepys regularly to his house, read him his verses, and drew on his own experience of the Navy in Charles I's time to explain how Pepys' own office, Clerk of the Acts, had been performed then. Bryant remarks that Pepys respected Slingsby as he could never respect other colleagues like Sir William Batten.
Death
Slingsby did not enjoy office for long: on 22 October 1661 Pepys noted that he was sick of ague and fever, and on 24 October he 'continued ill, which makes them all afeared for him'. He died on 26 October to Pepys' grief: 'he being a man that had loved me and had many qualities that made me love him above all the officers and Commissioners in the Navy'. Pepys was distressed that there was no memorial service, which he regarded as a deliberate slight on Slingsby's memory from other colleagues like Batten and Sir William Penn, who in Pepys' opinion had regarded Slinsgby's integrity as a check on their greed and ambition. Both Batten and Penn professed grief at Slingsby' death, but Pepys regarded them as a pair of hypocrites.
