Hacker was third son of Francis Hacker of East Bridgeford and Colston Basset, Nottinghamshire, by Margaret, daughter of Walter Whalley of Cotgrave. From the outbreak of the English Civil War Hacker vehemently supported the Parliamentary cause, though the rest of his family seem to have been royalists. On 10 July 1644 he was appointed one of the militia committee for the county of Leicester, the scene of most of his exploits during the Civil War, On 27 November 1643 he and several others of the Leicestershire committee were surprised and taken prisoners at Melton Mowbray by Gervase Lucas, the Royalist governor of Belvoir Castle. A month later Parliament ordered that he should be exchanged for Colonel Sands.