[3] Arundell entered Lincoln's Inn in 1633 and qualified as a lawyer in 1640, representing his elder brother in a legal case that same year.

[5] When the First English Civil War began in August 1642, Arundell joined the Royalist army and is thought to have served at Edgehill, as well as Lansdowne.

Historian and statesman Clarendon describes him as "a stout and diligent officer" although his primary role was raising taxes for the Royalist war effort.

He served under his father in the garrison of Pendennis Castle, which sheltered both Charles II of England and Queen Henrietta Maria during the war and was one of the last strongholds to surrender in August 1646.

[6] Since his elder brother John was killed outside Plymouth in 1644, Arundell inherited the family estates when his father died in 1654; he remained active in the Royalist underground during the Interregnum but managed to avoid arrest.