Mellor was born in Crewe, where his father was a Unitarian clergyman.
He attended Willaston School, an establishment set up to provide education for the sons of impoverished Unitarian ministers.
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 1937–38, but was sacked after falling out with Stafford Cripps over the latter's proposal for a Popular Front of socialist and non-socialist parties against fascism.
For the last ten years of his life, though married with a family, he conducted an affair with the young Barbara Castle.