New South Wales

Johnston made his first-class debut for New South Wales in a Sheffield Shield match against Tasmania in March 1999. He opened the bowling with Brett Lee who would make his debut for Australia later that year. Johnston's first wicket was that of Michael Di Venuto; he took one more wicket that day, but after play ended on the second day Johnston suffered a broken arm as the team coach drove off while he was climbing on. He played his only List A match for them in October that year, against Queensland. He played four Pura Cup matches for them in 2000 before his career took him to Ireland. New South Wales failed to notify Johnston that his contract had not been renewed and he was instead informed by a team-mate. He returned to club cricket where he represented Campbelltown, North Sydney, and Mosman, captaining the last.

Playing for Ireland

Johnston first played club cricket in Ireland in 1995 and returned for four more seasons, meeting his wife Vanessa, with whom he now has two children. He continued playing Sydney Grade Cricket for North Sydney Cricket Club, but qualified for an Irish passport and then-coach Adrian Birrell decided he might be a useful addition to the Irish side. He made his debut for Ireland on 5 May 2004 against Surrey in a C & G Trophy match that Ireland won, only their second win against a first-class county. He also played in the next match of the competition against Northamptonshire. Johnston's next appearances for Ireland were two matches against the West Indies in June, the second of which Ireland won. He then played against the MCC later that month, before making his first-class debut for Ireland against the Netherlands in the Intercontinental Cup. This was followed by the 2004 European Championship and a first-class match against Scotland. The main action for Ireland in 2005 was the 2005 ICC Trophy, hosted in both Northern Ireland and the Republic of Ireland. Johnston was in the Irish squad and they reached the final, losing to Scotland, but still qualifying for the 2007 World Cup.

Ireland captain

He was made Irish captain in time for the semi-final of the 2005 Intercontinental Cup against the UAE in Windhoek, Namibia. The match was drawn, but Ireland qualified for the final on bonus points. Ireland faced Kenya in the final, winning by six wickets. The following year started with several matches against English county sides before captaining Ireland in their first ever One Day International against England. He also played in the European Championship that year, which included ODIs against Scotland and the Netherlands.