Johnston wuxuu u saftay Munster 5kii Sebtember 2015, isagoo ka bilaabay ciyaartii furitaanka ee 2015 <unk> 16 Pro12
oo ka dhan ah Benetton. Bishii Janaayo 2016, waxaa la shaaciyay in Johnston uu heshiis la saxiixday Munster taas oo u arki doonta inuu ku biiro kooxda sare laga bilaabo xilli ciyaareedka 2016-17, marka hore heshiis horumarineed ka dibna, laga bilaabo 2017-18, heshiis buuxa. Johnston waxaa loo magacaabay Abaalmarinta Ciyaaryahanka Sanadka ee 2016 ee Munster Academy.