Johnston had a successful junior career and finished second on the Scottish Boys Order of Merit in 2010 and 2011.

[1] Johnston represented Scotland in the Men's Home Internationals in 2016 and 2017, both times losing to Ireland, and at the 2017 European Amateur Team Championship, where he lost to Sweden in the quarter final together with Craig Howie, Ryan Lumsden, Robert MacIntyre, Jamie Stewart and Connor Syme.

He won twice, at the Andalucía Costa del Sol Match Play 9 in May and the Kazakhstan Open in September.

Johnston also won a silver medal at the 2018 European Golf Team Championships at Gleneagles, together with a Great Britain made up of Connor Syme, Michele Thomson and Meghan MacLaren.

[5] In 2020 Johnston fired a career best 61 to lead by a single shot at the end of the first day of the Portugal Masters.