In 1649, Walter married Joan St John, also known as Johanna, who was a second cousin to him via his father. On 13 April1656, his nephew Sir John St John, 2nd Baronet, died, and Walter succeeded to the baronetcy, as Sir Walter St John, Baronet. He and Lady St John had thirteen children, but seven of them did not survive to adulthood. In 1700, Sir Walter signed a trust deed that led to the formation of a school which later became the Sir Walter St John's School of Battersea.
In 1656, Sir Walter also became a Member of Parliament for Wiltshire, and served as MP for that constituency or for the family borough of Wootton Bassett in the same county for most of the rest of his life.
Sir Walter died on 3 July1708, three years after the death of his wife.
References
Cobbett's Parliamentary history of England, from the Norman Conquest in 1066 to the year 1803 (London: Thomas Hansard, 1808) [1]