William Mellor (1888–1942) was a left-wing British journalist.
Mellor joined the Daily Herald in 191as a journalist, and was imprisoned during the First World War as a conscientious objector, returning to the Herald on his release. A Guild Socialist during the 1910s, he worked closely with G. D. H. Cole, founding the National Guilds League with him in 1915. He was a founder-member of the Communist Party of Great Britain in 1920, but resigned in 1924. He became editor of the Herald in 1926, succeeding George Lansbury when the Trades Union Congress took over the paper, and was fired in 1930 soon after Odhams Press took half-ownership with the TUC. He was the first editor of Tribune in 1937-38 and was sacked after falling out with Stafford Cripps over the latter's proposals for a Popular Front of socialist and non-socialist parties against fascism. For the last 10 years of his life, though married with a family, he conducted an affair with the young Barbara Castle.