Like his father, Theodore Junior was a professional soldier, first attested in this capacity when he was serving in the forces led by Algernon Percy, the Earl of Northumberland, in the Bishops' Wars in 1640.

At the outbreak of the English Civil War (1642–1651), Theodore sided with the Roundheads (Parliamentarians), despite his two brothers and his friend Richard Grenville being Cavaliers (royalists).

[8] On account of the absence of evidence for John's existence, the English Byzantininst Donald Nicol wrote in 1974 that the family's claim to descend from Thomas "must be held unproven".

[9] John Hall, the author of a 2015 biography on Theodore Junior's father, believes that it would be wrong to dismiss their descent on account of a single missing link.

[15] Theodore is next attested in 1640 (aged 31) as a lieutenant of the army led by Algernon Percy, the Earl of Northumberland, against the Scots in the Bishops' Wars.

Although his cause, time and place of death is unknown, by tracking the movements of his company, it can be assumed that he died of camp fever during the early stages of the long Siege of Oxford.

[21] Many Parliamentarian graves in Westminster Abbey were exhumed on the orders of Charles II after the restoration of the monarchy in 1660, with the corpses suffering symbolic executions and their heads being impaled on spikes.