He was born in Brondesbury Park, London, the sixth of seven children of the Canadian-born Quaker, Joseph Allen Baker and the Scottish-born Elizabeth Balmer Moscrip. His Father had moved to England in 1876 to establish a Manufacturing Business and served as a Progressive Party member of the London County Council from 1895 to 1906 and as Liberal Party member of the House of Commons for East Finsbury from 1905 to 1918.
Baker was educated at Ackworth School, Bootham School and then in the US at the Quaker-associated Haverford College in Pennsylvania. He studied at King's College, Cambridge from 1908 to 1912. As well as being an excellent student, obtaining a second in Part I history and a first in Part II economics, he was President of the Cambridge Union Society in 1912 and President of the Cambridge University Athletic Club from 1910 to 1912.
He was an academic early in his career. He was appointed vice-principal of Ruskin College, Oxford in 1914, and elected a fellow at King's College, Cambridge in 1915. During World War I, he organised and led the Friends' Ambulance Unit attached to the fighting front in France (1914–1915), and was then, as a conscientious objector from 1916, adjutant of the First British Ambulance Unit for Italy, in association with the British Red Cross (1915–1918), for which he received military medals from the UK, France and Italy.
In June 1915, Philip Baker married Irene Noel, a field hospital nurse in East Grinstead, subsequently adopting the hyphenated name Noel-Baker in 1921 by letters patent. His wife was a friend of Virginia Woolf. Their only son, Francis Noel-Baker, also became a Labour MP and served together with his Father in the Commons. Their marriage, however was not a success and Noel-Baker's mistress from 1936 was Megan Lloyd George, daughter of the former Liberal Party leader David Lloyd George, herself a Liberal and later Labour MP. The relationship ended when Irene died in 1956.
After World War I, Noel-Baker was closely involved in the formation of the League of Nations, serving as assistant to Lord Robert Cecil, then assistant to Sir Eric Drummond, the league's first secretary-general. He became the first Sir Ernest Cassel Professor of International Relations at the University of London from 1924 to 1929 and a lecturer at Yale University from 1933 to 1934. His political career with the Labour Party began in 1924 when he stood, unsuccessfully, for Parliament in the Conservative safe seat of Birmingham Handsworth. He was elected as the member for Coventry in 1929, and served as parliamentary private secretary to the Foreign Secretary Arthur Henderson.
Noel-Baker is the only person to have won an Olympic medal and received a Nobel Prize. He was a Labour member of parliament from 1929 to 1931 and from 1936 to 1970, serving in several ministerial offices and the cabinet. He became a life peer in 1977.
Noel-Baker lost his seat in 1931, but remained Henderson's assistant while Henderson was President of the World Disarmament Conference in Geneva in 1932 to 1933. He stood for Parliament again in Coventry in 1935, unsuccessfully, but won the Derby by-election in July 1936 after the sitting Derby Member of Parliament J. H. Thomas resigned. When that constituency was split in 1950, he transferred to Derby South.
Noel-Baker became a member of the Labour Party's National Executive Committee in 1937. On 21 June 1938, Noel-Baker, as M.P. for Derby, in the run up to World War II, spoke at the House of Commons against aerial bombing of German cities based on moral grounds. "The only way to prevent atrocities from the air is to abolish air warfare and national air forces altogether."
In the coalition government during the World War II he was a parliamentary secretary at the Ministry of War Transport from February 1942, and served as Minister of State for Foreign Affairs after Labour gained power following the 1945 general election, but had a poor relationship with the Foreign Secretary, Ernest Bevin. Noel-Baker moved to become Secretary of State for Air in October 1946, and then became Secretary of State for Commonwealth Relations in 1947 and joined the cabinet.
Noel-Baker was the minister responsible for organising the 1948 Olympic Games in London. He moved to the Ministry of Fuel and Power in 1950. In the mid-1940s, Noel-Baker served on the British delegation to what became the United Nations, helping to draft its charter and other rules for operation as a British delegate. He served as Chairman of the Labour Party in 1946–47, but lost his place on the National Executive Committee in 1948, his place being taken by Michael Foot. An opponent of left-wing Bevanite policies in the 1950s, and an advocate of multilateral nuclear disarmament, rather than a policy of unilateral disarmament, he received the Nobel Peace Prize in 1959. In 1979, with Fenner Brockway, he co-founded the World Disarmament Campaign, serving as co-Chair until he died, and was an active supporter of disarmament into the 1980s.
Noel-Baker stood down as the MP for Derby South at the 1970 general election, at which he was succeeded by Walter Johnson. His life peerage was announced in the 1977 Silver Jubilee and Birthday Honours and he was raised to the peerage 22 July 1977, as Baron Noel-Baker, of the City of Derby, having declined an appointment as a Companion of Honour in the 1965 New Year Honours. He was President of the International Council of Sport Science and Physical Education from 1960 to 1976.