The following year he settled into the side and, playing alongside the likes of John Charles and John Toshack, he finished as the club"s top scorer with a total of 23 goals.
He played more than 150 matches in the English Football League. He began his football career as a junior at Cardiff City, turning professional in 1964 and making his debut at the end of the 1964-1965 season at the age of 17. He was part of Cardiff"s 1965 Welsh Cup-winning team
In March 1967, Johnston played in a benefit match for victims of the Aberfan disaster against Arsenal and scored twice against the London club, which persuaded them to offer £20,000 to Cardiff to sign him.
He made his debut for Arsenal on August 19, 1967, against Stoke City and made 17 appearances in his first season, 1967-1968. Out of the Arsenal first team, he joined Second Division club Birmingham City in the summer of 1969, for a fee of £30,000.
In total he made 25 appearances and having scored 3 goals for Arsenal. Signed as a replacement for Fred Pickering, he failed to hold down a first-team place.
A period on loan at Walsall immediately preceded a £6,000 move to Fulham, where he spent two years.
A season with Hereford United followed, and one more with Newport County, before, at the age of just 26, he dropped out of league football. He settled in Cardiff and worked for a marine engineering company. with Cardiff City.