 Through his father, he was third cousin to George Onslow, later the Earl of Onslow.Middleton was put into Parliament for Rye at the 1774 election to hold the seat until George's son Tom should come of age to take the seat.No political activity on Middleton's part is recorded, and on 20 April 1775, he took the Manor of East Hendred to leave the seat and allow it to pass to Tom.