 Slanning was the son of Gamaliel Slanning of Hele, Devon, by his wife and cousin Margaret Marler, daughter of Edward Marler of Crayford, Kent.The Slanning family is first documented in 1538 and spanned nine generations until the extinction of the male line in 1700.The Slannings were granted or acquired land in Bickleigh, Walkhampton, Maybury, and Roborough, all near Plymouth.