Born in Newbuildings, County Londonderry, Middleton studied Computer Science at the University of Ulster, and also obtained a diploma in Civic Leadership and Community Planning from the same institution.

Prior to becoming an MLA, he was a youth worker and also worked as an assistant to Foyle MLA William Hay and later Maurice Devenney, who was himself co-opted to replace William Hay in October 2014 upon the latter's retirement after being elevated to the House of Lords.

[2] Middleton was first elected to Derry City Council in 2011 at the age of 20, replacing William Hay by co-option.

[3] In 2014 he was elected to the new Derry and Strabane 'super council' and was appointed group leader of the DUP councillors.

[4] Shortly after joining the assembly, it was announced that Middleton would be the DUP's candidate for the Foyle constituency in the United Kingdom general election, eventually finishing a distant third with 12.4% of the vote.