Johnstone played alongside Jardine in the Gers side that beat Moscow Dynamo at the Nou Camp in 1972 to win the European Cup- Winners' Cup.
And he was distraught to learn the former Scotland full-back, who had fought a brave fight against cancer in the last couple of years, had tragically died aged 65.
He described the two-time Scottish Player of the Year, who played almost 800 games for the Ibrox club, as "a great man and a true Rangers man".
Johnstone said: "I was lucky to play alongside Sandy for probably my entire career. He was just a fantastic player. When you talk about great full-backs he has to be included.
"In my time it was Danny McGrain and Sandy Jardine. He was a very intelligent player, one of the first full-backs who went upfield and attempted to score goals. On top of all that, he was a very nice man.
"I knew he was ill, I knew the cancer had come back. But he was still trying to help Rangers. I saw him at Ibrox not long ago chatting to folk. This is just so sad. He was one of the good guys."
After a successful spell playing and coaching at Hearts alongside his old Rangers team-mate Alex MacDonald, Jardine returned to Ibrox in the 1990s. He worked at the Gers for many years in a variety of roles and became a figurehead for the club when they went into administration in 2012.
Johnstone believes Rangers fans owe him a great deal for his years of loyal service both as a player on the park and as a dedicated servant off it.
He said: "All Rangers fans and Rangers Football Club should be indebted to him. He was involved with Rangers for many, many years as a player and then behind the scenes at the club.
"When they went into administration he took it upon himself to stand up for the club. Rangers didn't have many figureheads at that time, but he was one.
"Sandy was a true Rangers man. He lived for the club. This is just so sad ... he was one of the good guys."
Former Ibrox team-mate and Scottish football legend Sir Alex Ferguson added to the tributes and said: "From Cathy and I, this is some of the worst news we have heard. Sandy was a noble and courageous man. The respect he is held in at Rangers is immense.
"He was one of the greatest players ever to wear the jersey. To Shona and family, we express our sympathy and sadness."
Former Rangers manager Walter Smith said: "He was a gentleman and someone I always felt privileged and honoured to know."
Celtic chief executive Peter Lawwell said: "This is absolutely devastating news and our thoughts and prayers are very much with Sandy's wife Shona, his children Steven and Nicola, and the wider family."