Though only 24, in August 1844 Middleton gained his first contract and was asked to design Cleveland Lodge, an impressive mansion situated directly beneath Roseberry Topping.
He moved to Cheltenham in 1859 where he designed five Anglican churches, which were All Saints', Holy Apostles, St. Mark's, St. Philip & St. James and St. Stephen's Churches.
This move to Cheltenham brought Middleton to the attention of a number of landed gentry in Wales.
Middleton married Maria Margaret Pritchett, daughter of James Pigott Pritchett, architect of York, and his first wife Peggy Maria Terry.
Their only surviving child John Henry Middleton (1846–1896)[1] was appointed director of the Victoria and Albert Museum in 1893.