John Robert Mortimer was born at Fimber in the Yorkshire Wolds on 15 June 1825, the eldest of three children of a local farmer; he received a village school education at Fridaythorpe. In adult life he operated as a corn merchant, moving to the nearby larger town of Driffield in 1869; in the same year marrying Matilda née Mitchell, daughter of the vicar of Sancton and of Holme on the Wolds. His business included trade in seed, manure and fertiliser, and included malt kilns and a brewery as assets.