Johnson, on the disabled list since May 21 with right shoulder inflammation, previously threw off a mound in late June.
Johnson threw 21 fastballs off the Sun Life Stadium bullpen mound before Monday night's game against the New York Mets. The plans were for him to throw again Wednesday and Saturday.
"Today was just get out there, and get full extension," Johnson said. "I have no doubt I'll be ready for next season."
If all goes well, Johnson said he could pitch in a simulated game before the season ends. He was 3-1 with a 1.64 ERA in nine starts this season.
Also, Marlins shortstop Hanley Ramirez will undergo a CT Scan to determine the type of surgery he'll face to repair his left shoulder. The shoulder has popped out of its socket several times, the most recent being last Tuesday night in a rehab game with Class A Jupiter (Florida State League).
No surgery date had been set, but Ramirez said he expected to be in the next couple of weeks.
Ramirez also underwent left shoulder surgery in October 2007. He batted .243 with 10 homers and 45 RBI in 92 games this season.