Simon Neville Llewelyn Marsden was the younger son of Sir John Denton Marsden, 2nd Bt, and his wife Hope (née Llewelyn). The baronetcy was created in 1924 for a previous John Marsden, owner of a substantial fishing fleet in Grimsby. Marsden attended Ampleforth College in North Yorkshire, as well as the University of Sorbonne. From 1969, he worked as an assistant to Irish photographer Ruan O'Lochlainn, whose wife, Jackie Mackay, was a master printer from whom Marsden learned the skills of the darkroom.