He served in the South African government from 1905–10 after which he returned to England to found and edit the Round Table Journal. In 1916, he was appointed David Lloyd George's private secretary and was active in the Paris Peace Conference. For these services he was appointed a Member of the Order of the Companions of Honour (CH) in March 1920.
He was a member of what was called "Milner's Kindergarten". This was more a group of colonial officers who deemed themselves reformist than an actual political faction. They believed the colonies should have more say in the Commonwealth. In the terms of the era they were liberal, but in modern terms they might be deemed right wing as most of them only had interest in elevating the status of white colonials, rejected independence, and had a paternalistic view of non-whites. Philip Kerr became more liberal on these issues than them, admiring Mohandas Gandhi and trying, if not entirely succeeding, to be more progressive than them on racial issues.
He came from an aristocratic family who were staunch members of the Roman Catholic Church. He himself considered becoming a priest or monastic at times, but in adulthood he became disillusioned with the faith. His close friendship to Nancy Astor led to their both converting to the Church of Christ, Scientist together. The reaction of his family to this eventually led to his support of Anti-Catholicism. A confirmed bachelor having never been romantically linked with any female, he left no heirs and the marquessate was inherited by his first cousin, Peter Kerr.
Lord Lothian was best known in the United States for having aided Washington Post owner Eugene Meyer scoop the world on reporting Britain's King Edward VIII's relationship with Wallis Simpson, eventually leading to Edward's abdication of the crown.
Speaking on 24 June, 1933, at Gresham's School, he said "There probably never was a time of more uncertainty in the world than today. Every kind of political and economic philosophy is seeking approbation, and there is every kind of uncertainty about social and personal habits.
He was later British Ambassador to the United States of America, from 1939 to 1940. He felt initial sympathy for Germany over the Treaty of Versailles and so at first he favored appeasement, but later abandoned the idea. Devoted to the very end to the religion to which he had converted, he died having refused medical treatment as a Christian Scientist.