Born at Palmers Green, north London, to working-class parents Thomas James Nathaniel Gregg, a Post Office mail sorter, and Elizabeth Janette (née Kuttner),[1] Gregg was attracted to socialism during her schooldays, joining the Labour League of Youth and Independent Labour Party, and addressing meetings from a coal cart, later from Speakers' Corner in Hyde Park.
[2] Gregg was offered a lectureship at Hillcroft College, Surbiton, south-west London, but joined the Ministry of Supply when the Second World War began, and was posted to Warwick.
From 1946, they were in charge of Holywell Manor, Oxford, an annexe of Balliol, housing fifty undergraduates; although finding housework dull, Gregg was "a dashing hostess".
During this time, Gregg daily researched at the Bodleian Library; her first book, A Social and Economic History of Britain 1760-1950, became a standard reference work, and her "definitive" biographies of King Charles I and the regicide Cromwell were well-regarded, C. V. Wedgwood, herself an expert on 17th century English history and author of many books on the period, calling the former "the fullest and most carefully compiled that we are ever likely to have".
[2] Gregg's published works concentrated on the period of the English Civil Wars of the 17th century and the history of social life in Britain.