The Manchester City manager clashed angrily with Liverpool skipper Steven Gerrard after Wednesday night's 1-0 Carling Cup defeat.
Still fuming at Kompany's appeal being turned down on Tuesday, Mancini feels there was a clear lack of consistency in Johnson not even getting booked for his two-footed challenge on Joleon Lescott at the Etihad Stadium on Wednesday night.
"It was worse," he said.
"This tackle was worse. Gerrard came to me and said I said something.
"It was not for Johnson. It is for the tackle. This tackle was worse than Vinny's. Everyone can see it."
Mancini clashed with Gerrard twice, the second particularly fractious, although the Liverpool goalscorer was unrepentant.
"It surprises me because he had a go at Wayne Rooney. Now he's had a go at one of ours," Gerrard told BBC1, in response to comments from Mancini claiming the England striker had influenced the decision to dismiss Kompany.
"Steven Gerrard can say what he wants," Mancini responded.
"I said what I think. That is what I am used to doing.
"It is not important what Steven Gerrard or the other players have said."
The incident completed a bad night for Mancini and his team, who were suffering back-to-back home defeats for the first time since February 2008.
Kompany's stand-in Stefan Savic had a particularly difficult time, struggling to contain Liverpool forward Andy Carroll and conceding the penalty that allowed Gerrard to ram home the winner when he caught Daniel Agger.
That Savic is expected to retain his place for the Premier League trip to Wigan next Monday hardly inspires confidence, although Mancini is looking on the bright side.
"Savic needs to get experience," he said.
"It is normal. He is young. Against a striker like Carroll maybe he could have some difficulties.
"But in the second-half it went better."
Mancini confirmed he expects to know within the next 48 hours the extent of the ankle injuries that kept David Silva out tonight and forced Mario Balotelli off towards the end of the first half.
After losing three times in their last four games, City are in their toughest period of the season and with Tottenham to face before the end of the month, in addition to the decisive semi-final at Anfield just afterwards, it is not getting any easier, with both Yaya and Kolo Toure absent on African Nations Cup duty with the Ivory Coast.
"We have just played two difficult games," he said.
"I knew this before, when I said that January will be an important month for us because we lose three or four important players."