In 1908, Comyns Carr was called to the Bar at Gray's Inn. He became a King's Counsel in 1924, a Bencher of the Inn in 1938 and eventually Treasurer in 1951. Comyns Carr's reputation as a barrister was confirmed in a libel action brought by Horatio Bottomley against an associate named Reuben Bigland. Carr's cross-examination of Bottomley and another key witness destroyed his case and was instrumental in Bottomley's eventual imprisonment on charges of fraud and his expulsion from the House of Commons. Comyns Carr later began to specialize in the law relating to local taxation and as a result of appearing in landmark rating appeals he was engaged as counsel to government departments. He also became an expert in the subject of national insurance. Much later Comyns Carr was a prosecutor in trials of German and Japanese war criminals, and he was knighted for this work in 1949.
Comyns Carr's expertise in National Insurance led him to co-author a book on the subject in 1912 to which David Lloyd George wrote the preface. He was a member of the Liberal land inquiry committee of 1912 and also sat on the land acquisition committee in 1917. His ambition to become a Liberal Member of Parliament led Comyns Carr to stand for Parliament on eleven occasions in all. He first stood for election in 1918 in St Pancras South West against a Conservative opponent who had received the Coalition Coupon and fought the same seat again in 1922. At the 1923 general election Comyns Carr had his only success, becoming Liberal MP for Islington East turning a Unionist majority of nearly 4,000 into a Liberal majority of 1,632 but he lost the seat at the general election of 1924 like many other Liberals swept away as British politics seemed to be reverting to its traditional two party model. In 1928, he was Liberal candidate at the by-election for the constituency of Ilford and fought the seat again in the general election of the following year. In 1930, Comyns Carr published an influential and controversial booklet, Escape from the Dole, which gained him significant public attention. In it he queried the policy of spending large sums of money supporting the unemployed when the government could be investing in providing work for them. He then challenged Winston Churchill in his constituency at Epping in the 1931 general election and in 1935 he suffered his heaviest defeat ever at Nottingham East. In June 1936 he was elected to serve on the Liberal Party Council. He stood again in 1945 when he lost at Shrewsbury. In October 1945 he was a candidate at another by-election, this time in the City of London.
