Armstrong was born in Newcastle upon Tyne to an Irish father and Nigerian mother, and moved to London at age three. He was raised by foster parents in Wales, where he began playing as an amateur goalkeeper in local leagues, before leaving school and giving up on football aged 16. A friend reintroduced him to the game by bringing him to Llay Welfare in the Welsh National League, while he packed burgers during the day for £30 a week. He gained the attention of local professional club Wrexham.