He is Lord Steward of the Royal Household and Personal Secretary to Their Majesties, as well as a former Metropolitan Police Commander.

[5] Rosslyn joined the Metropolitan Police Service in 1980 on the recommendation of his third cousin Lord Strathnaver,[2] a former detective and heir apparent of the 24th Countess of Sutherland.

[7] In March 2014, Lord Rosslyn was appointed as Master of the Household to The Prince of Wales, and The Duchess of Cornwall at Clarence House.

[8] On 22 February 2023, he was appointed Lord Steward of the Household, succeeding the 17th Earl of Dalhousie, and Personal Secretary to Their Majesties by Charles III.

[10] He married Helen Watters in 1982 and they have two sons and two daughters, including actress Lady Alice St Clair-Erskine.