The Evening Times columnist revealed his shock at the 63-year-old's diagnosis, but branded Jardine a "fighter" who would do everything possible to beat the disease.
Jardine, who still works for Rangers, revealed at the weekend he is currently being treated for cancer.
Football pundit Johnstone said: "Sandy pulled me aside last Saturday at Ibrox and told me he had cancer.
"He just told me straight out – I was in total shock. When he told me, he had a slight tear in his eye, as you can expect.
"But he's a fighter and the Rangers family will be behind him all the way. Sandy is one of the fittest men I know, he always has been.
"It's just been such a massive shock."
Jardine played more than 500 games for Rangers, mostly at right back, during a 17-year spell at the club from 1965 to 1982. He was part of the Ibrox side that won the 1972 European Cup-Winners' Cup.
Since February, Jardine has been heavily involved with the Rangers Fighting Fund, raising cash for the club throughout their recent financial difficulties.
Johnstone added: "For the last 10 months, Sandy has really been there for Rangers when they've needed him most. Now, we're all praying that he will come through this."
"Obviously he's still in hospital at the moment, but everyone is thinking about him."
Former Scotland defender Jardine, said: "I would like to thank everyone for their best wishes and kind support.
"As you can appreciate, this is a difficult time for me and my family and I ask that you respect our privacy."
In a statement, Rangers said: "The club will support Sandy and his family in any way it can throughout his treatment."