Medel was the best player on the pitch during the 1-0 win over Swansea in the Barclays Premier League's first meeting of the Welsh rivals.
The victory highlighted Mackay's value to Cardiff after a run of recent events and revelations, including the removal of his head of recruitment Iain Moody, threatened to undermine his position and alienated him from controversial owner Vincent Tan.
Chile international Medel had come under scrutiny after Swansea full-back Angel Rangel claimed Cardiff had spent too much - having parted with £11million in the summer - to sign him, and questioned his temperament.
But the 26-year-old put the lie to those suggestions with a commanding performance, breaking up Swansea attacks and distributing the ball effectively with a minimum of fuss.
"He was immense from first to last minute," said Mackay.
"In the first half it was more about organising and collecting second balls, but in the second half it was about having the courage to get on the ball at any point and get the game flowing and his amount of time on the ball in the second half must have been huge.
"Having said that, I am loath to pick out one guy, as I had a performance from my whole team and subs, a mature performance considering what they have had to endure."
And Mackay explained how Medel's contribution had been key to his tactical masterplan of upsetting Swansea's passing rhythm.
"We talked about how Swansea play and how they flood certain areas to get their passing game going," he said.
"The ball being passed between the two centre-backs all day is fine and in the first half we made sure we filled and pressed hard in certain areas when the ball was put in that area to disrupt their system.
"As the game went on we knew spaces would open up and we could exploit certain areas and we knew if we got Medel on the ball he could orchestrate the whole game and he was able to do that and find our guys in areas we could exploit."
Derby success was particularly sweet for Craig Bellamy, who was the only player from Cardiff in the Bluebirds team.
And Mackay felt the 34-year-old's previous experience of the fixture had been a key asset.
"When it came to the derby, I felt there was experience there," said Mackay.
"He has been mature over the last two years since I have been here, even more so this year as he has called time on Wales and he knows the sun is going down on his career.
"But he has been immense this season on and off the field, whether he has not been picked, whether he has been starting, used as a substitute or being asked to train hard.
"Having him on the field in that derby was something I really wanted, we spoke about it during the week and I think his experience showed.
"He got a second wind with 10 minutes to go which I hadn't seen before and I don't think he had seen either.
"It's one of those where he showed experience through the day."