Born at Chislehurst, Kent, on 25 Nov 1905, Patrick Arthur Devlin was the eldest of three sons and the second of five children of William John Devlin (d. 1932), architect, and his wife, Frances Evelyn (Fanny), née Crombie. His father, William, came from a Catholic family from Tyrone in Ulster, but had left Ireland for London to train as an architect. His mother, Frances, was a member of a wealthy Aberdeen family of cloth manufacturers, who were protestants. His father's practice did not prosper and in 1909, the family moved from Chislehurst to Aberdeen.

Devlin was sent to school at Stonyhurst College (1914–1922) and in 1925 committed himself to a career at the bar. Devlin had already, on 12 Feb 1932, married Madeleine Hilda Oppenheimer (1909–2012), younger daughter of Sir Bernard Oppenheimer, the diamond magnate. They had six children, four boys and twin girls, and, in due course, dozens of grandchildren.

Devlin was Chairman of a Commission of Enquiry into the declaration by the governor of a state of emergency in Nyasaland (Malawi). The commission concluded that there were grounds for declaring the state of emergency but that the supposed plot by Dr Hastings Banda and others to murder the governor, his officials, and all the Europeans in Nyasaland (Malawi) was a figment of an informer's imagination. However, despite criticisms of the commission's conclusions, the Prime Minister promoted him to the Court of Appeal in 1960, and to the House of Lords (as Baron Devlin of West Wick) in 1961.

For a full biography, see The National Archives.