John Tompkins was born on November 1, 1834 at Looke in the parish of Abbotsbury, Dorset, England the fifth of eight children of Richard Tompkins and Ann Churchill. Despite being born in Abbotsbury, John was baptised in the parish church at Puncknowle, which was physically much closer to the family home.
The Sydenham family had owned and lived on the estate at Wynford Eagle near Maiden Newton, Dorset for well over 100 years by the time William Sydenham married Mary Jeffery, the daughter of John Jeffery of Catherston in 1611. William himself, having inherited the estate at about the age of fourteen on his grandfathers death in 1607, his own father having died before he was one year old. In this quiet rural backwater William and Mary were to have ten children, and must have taken great pride in their five sons who reached manhood.