His elder brother was John St Clair, 17th Baron of Roslin who died in 1690 and whom he succeeded.

[1] He redeemed the estate of Roslin and was an apprentice in London after which he went to France where he spent time with Mr Monteith who was the author of The Troubles of Great Britain, then settled with the Knight of Malta, Great Prieur of France.

He later entered into service with the Archbishop of Corinth to who he dedicated his book Troubles of Brittany.

He later served Lord Rutherford the Viscount Teviot as his secretary in Dunkirk and at Tangier in Africa.

[3] James St Clair, 18th Baron of Roslin married to Jean, daughter of Henry Spotswood the Sheriff of Dublin.