Upon the death of his unmarried uncle in 1906, his father became the Earl of Mansfield and young Mungo was styled Lord Scone.

[2] His paternal grandparents were William Murray, Viscount Stormont (heir apparent to the 4th Earl of Mansfield) and Emily Louisa MacGregor (a daughter of Sir John Murray-Macgregor, 3rd Baronet).

His uncle, William Murray, 5th Earl of Mansfield, who was a friend of King Edward VII, was known as "The most eligible bachelor" in London, and threw lavish parties at Kenwood House.

His maternal grandparents were Rear-Admiral Sir Malcolm Murray-MacGregor, 4th Baronet and Lady Helen Laura McDonnell (a daughter of the 4th Earl of Antrim).

[2] He was active in the extreme anti-Catholic Scottish Protestant League before breaking with them following the 1929 United Kingdom general election.