Middleton’s family, who had been settled at Belsay in Northumberland since the thirteenth century, had represented the county since early in the fifteenth. After the Restoration they were one of the few big landowning families to remain Presbyterians. During Middleton’s minority his chief trustee was the Presbyterian minister at Belsay and his mainstay at elections was the Presbyterian vote, which was especially strong among the farmers.
On coming of age Middleton was returned as a Whig for the county, which he continued to represent for the rest of his life. He usually supported the Government but voted against them on the excise bill in 1733 and the place bill of 1740. In May 1742 he was one of the members on the court list who were chosen as commissioners of public accounts under an opposition bill which was thrown out in the Lords.
According to family tradition Middleton ‘was always borrowing money and always in debt’,
