[3] Fourth Division club Darlington's initial attempts to sign Johnstone in December broke down,[4] but at the end of the season, after he took no further part in Carlisle's second successive promotion, the move took place.
[3] Johnstone made his debut on 23 August 1965, 78 minutes into a 2–0 defeat at Lincoln City, replacing Les O'Neill who had twisted a knee.
According to the Journal, he "had particularly bad luck with two drives, one of which hit the bar from 25 yards (23 m) and the other England international goalkeeper Tony Waiters did well to palm away".
[5] Although his goalscoring dried up, he finished with nine goals from 25 appearances as Lol Morgan led Darlington to runners-up spot in the 1965–66 Fourth Division, which gave them their first promotion since the 1920s.
He was a member of the Shields side that eliminated two Football League opponents – Bradford Park Avenue[12] and Oldham Athletic, for which he was marked by 1966 World Cup-winner Ray Wilson[13] – to reach the third round of the 1969–70 FA Cup, in which they faced a Queens Park Rangers side including the likes of Rodney Marsh and Terry Venables.