In July 1643, he was created Earl of Sunderland in the Peerage of England, but he was killed at the Battle of Newbury in September of the same year.

He later gained great distinction as a statesman and notably served four times as Secretary of State for the Southern Department.

The dukedom of Marlborough was allowed to descend to the Duke's daughters and their sons after a special Act of Parliament was passed in 1716.

The barony of Spencer of Wormleighton and earldom of Sunderland have remained subsidiary titles of the dukedom ever since.

John Spencer, fourth son of the third Earl of Sunderland by his second wife, succeeded to the family estates in Northamptonshire in 1733 after his elder brother inherited the dukedom of Marlborough.