He left science for the law, and in 1913 was called to the bar by the Middle Temple. He served in the First World War as a Red Cross ambulance driver in France, and then managed a chemical factory producing armaments. He remained a barrister during the 1920s, where he specialised in patent cases, and was reported to be the highest paid lawyer in England. He was appointed a King's Counsel in 1927. Cripps was a member of the Church of England and in the 1920s became a leader in the World Alliance to Promote International Friendship through the Churches, as his father had been. In 1923 to 1929 Cripps was the group's treasurer and its most energetic lecturer. At the end of the 1920s Cripps moved to the left in his political views, and in 1930 he joined the Labour Party. The next year, Cripps was appointed Solicitor-General in the second Labour government, and received the then customary knighthood. In 1931, Cripps was elected in a by-election for Bristol East. During this time in parliament, he was a strong proponent of Marxist social and economic policies, although he had strong faith in evangelical Christianity, and did not subscribe to the Marxist rejection of religion. In the 1931 general election, Cripps was one of only three former Labour ministers to hold their seats alongside the party leader George Lansbury and deputy leader Clement Attlee. In 1932 he helped found and became the leader of the Socialist League, which was composed largely of intellectuals and teachers from the Independent Labour Party who rejected its decision to disaffiliate from Labour. The Socialist League put the case for an austere form of democratic socialism. He argued that on taking power the Labour Party should immediately enact an Emergency Powers Act, allowing it to rule by decree and thus "forestall any sabotage by financial interests,” and also immediately abolish the House of Lords. In 1936, Labour's National Executive Committee dissociated itself from a speech in which Cripps said he did not "believe it would be a bad thing for the British working class if Germany defeated us". Cripps also opposed British rearmament:  Cripps was an early advocate of a United Front against the rising threat of fascism and he opposed an appeasement policy towards Nazi Germany. In 1936 he was the moving force behind a Unity Campaign, involving the Socialist League, the Independent Labour Party and the Communist Party of Great Britain, designed to forge electoral unity against the right. Opposed by the Labour leadership, the Unity Campaign failed in its intentions.