She made the call for a timetable to be set up to outline how the powers could be made stronger to mark yesterday’s International Right to Know Day.
Ms Johnstone has written to Deputy First Minister Nicola Sturgeon, whose new cabinet responsibilities include FoI.
Ms Sturgeon recently said she was happy to discuss the extension.
She said: “In the decade since Freedom of Information came in, public accountability has clearly been eroded.
“There are now more than 130 private and arms-length organisations delivering public services such as leisure and sports facilities and they are not covered by FoI.
“I have written to the Deputy First Minister, welcoming her comments and asking her to outline why she feels a delay is appropriate and if she would be willing to set a timescale for the start of a discussion.
“There is a keen public appetite to look at extending FoI sooner rather than later.”