Married (1761) to Johanna Sara Bicker (1731-1801). He rose to the rank of Vice-Admiral in the Dutch navy.
This ID of the Captain Reynst mentioned by Boswell is based on the initial identification in Boswell in Holland. However, it doesn't seem to be entirely certain that Pieter Reynst is indeed the Reynst who Boswell met at the Spaen's. In a footnote to a letter from Baroness von Spaen to Boswell dated July 22, 1766, and quoted in The General Correspondence of James Boswell 1766-67, a Jean Lucas Reynst (1730-92) is mentioned, who was also an army officer in 1764 and who later became Colonel (1774) and Major-General (1787). It is not impossible that Boswell knew them both, and that they were both friends of the Spaens.
Boswell met him at Baron von Spaen's in the Hague on December 21, 1763 - it is not certain whether they had met before, as Reynst was also a friend of Belle de Zuylen's and had some acquaintances in Utrecht society, which Boswell was, at the time, a part of. Boswell visited Reynst at The Hague around April 19, 1764.