[1] He won the 2015 Munster Schools Rugby Senior Cup with Rockwell College, leading the side from fly-half in their 23–13 victory against Ardscoil Rís.
[7] On 10 December 2016, Johnston scored 10 points, all from the tee, as Munster A beat Rotherham Titans 35–24 away from home in Clifton Lane in Round 3 of the 2016–17 British and Irish Cup.
[9] Johnston made his competitive senior debut for Munster when he came off the bench as a replacement for Tyler Bleyendaal in the 50–14 away win against Zebre in a 2016–17 Pro12 fixture in the Stadio Sergio Lanfranchi, Parma on 25 March 2017.
Johnston had been a late call-up to the Munster bench after his older brother, David, replaced the injured Andrew Conway in the starting line-up.
[17][18] Johnston joined English RFU Championship club Ealing Trailfinders on loan in April 2021, where his older brother David is also a player.
He contributed 10 points in Ireland's historic 33–24 win against New Zealand U20, the sides first against their U20 rivals, but had to leave the field with a dislocated shoulder late in the first-half.