 Johnston was born and raised in Kilmarnock with his twin brother Ben (who became the drummer for Biffy Clyro), and his younger brother, Adam Johnston (who was Biffy Clyro's drum tech).The first gig he ever attended was Rancid at Glasgow Barrowlands in 1995, when he was fifteen.Having previously played bass with schoolfriend Simon Neil and brother Ben Johnston in a band called Skrewfish in 1995, the trio moved to Glasgow, and were soon discovered by manager Dee Bahl, and then signed to Beggars Banquet, in 2001.