Mark Lyall Grant was born 29 May 1956, son of Major-General I. H. Lyall Grant. He was educated at Eton and Trinity College, University of Cambridge (MA Law). In 1986, he married Sheila Jean Tresise, having one son and one daughter.

Lyall Grant was called to the Bar, Middle Temple, in 1980, but chose to join the Foreign and Commonwealth Office instead. His diplomatic career included the following posts: Urdu language training, SOAS and Lahore, 1981; Second Secretary, Islamabad, 1982-85; FCO, Southern European Department and Resident Clerk, 1985-86, European Union Department, Internal, 1986-87; Private Secretary to Minister of State, FCO, 1987-89; First Secretary, Paris [France], 1990-93; FCO, European Union Department, External, 1993; seconded to European Secretariat, Cabinet Office, 1994-96; Deputy High Commissioner and Consul General, South Africa, 1996-98; Head, EU Department (Internal), FCO, 1998-2000; Africa Director, FCO, 2000-03; High Commissioner to Pakistan, 2003-06; Political Director, FCO, 2007-09; Permanent Representative to the UN, 2009-15; National Security Adviser, 2015-17.