Johnston made his debut for Munster on 5 September 2015, starting in the opening game of the 2015–16 Pro12 against Benetton. In January 2016, it was announced that Johnston had signed a contract with Munster that will see him join the senior squad from the 2016–17 season, firstly on a development contract and then, from 2017–18, on a full contract. Johnston was nominated for the 2016 Munster Academy Player of the Year Award.