[1] By the time Thomas was ten years old, the English Civil War had begun between Parliament and King Charles I over whose authority took precedence over the other.
While his father was away, Thomas's uncle, Sir Simon Fanshawe, married the Katherine Walter, the widow of Knighton Ferrers, who was a wealthy neighboring landowner.
According to popular legend, Katherine became the "Wicked Lady", a female highwayman who terrorised the county of Hertfordshire.
For his service to the new king, Charles II, he was created a Knight of the Bath in 1661 upon the Restoration and became an active member for Hertford in the Cavalier Parliament from 1661 to his death.
Lord Clarendon later wrote to Grimston deploring the 'unwarrantable folly' of his old friend Fanshawe adding that his 'passion and animosity did the king no service'.