John was a younger brother of Henry Sinclair, Bishop of Ross, and their sister Margaret was married to Robert Lauder, of The Bass (d. June 1576).
Afterwards, as Rector of Snaw, John Sinclair was admitted as an Ordinary Lord in the Court of Session on 27 April 1540.
He was afterwards appointed by King James V, confirmed by a Bull of Pope Paul III dated 27 August 1542, as Dean of the Church of St. Mary of Restalrig, a valuable benefice stated to be £60 sterling.
As Dean of Restalrig, he had the honour of marrying Mary, Queen of Scots to Henry Stewart, Lord Darnley in Holyrood Abbey on 29 July 1565.
There are two manuscript copies of this work preserved in the Advocates' Library in Edinburgh, and one of them is stated by Ruddiman, in an Introductory Note, to have been transcribed from the original MSS, then in the possession of Lord Pitmedden.