Johnston akaita kutanga kwake kweMunster musi wa 5 Gunyana 2015, achitanga mumutambo wekuvhura we2015 <unk> 16 Pro12
vachirwa naBenetton. Muna Ndira 2016, zvakaziviswa kuti Johnston akanga asaina chibvumirano neMunster iyo ichamuona achibatana nechikwata chevakuru kubva mumwaka wa 2016-17, kutanga pachibvumirano chekuvandudza uyezve, kubva muna 2017-18, pachibvumirano chakazara. Johnston akasarudzwa nokuda kwe2016 Munster Academy Player of the Year Award.