Few had given the Welsh club a chance of pulling off an upset in their first Barclays Premier League home game, and they fell behind to Edin Dzeko's stunning strike early in the second half.
Alvaro Negredo got a goal back for City in stoppage time, and spurned a last-gasp chance for an equaliser, but it was not enough as manager Maneul Pellegrini was given a rude awakening after their handsome opening win over Newcastle.
Campbell's career has been heavily affected by injuries since making his breakthrough at Manchester United, but his pace constantly troubled Joleon Lescott and his stand-in defensive partner Javi Garcia.
His manager believes performances of this quality mean he could soon be adding to his solitary England cap.
Mackay said: "I thought Fraizer Campbell gave their whole back four problems.
"He is someone I was delighted to get when we managed to get him in from Sunderland for B£600,000. I thought it was an incredible bit of business on our behalf and he hit the ground running.
"He needed to get fit and he needed to get sharper and he needed to regain belief in his own ability and that is what we gave him.
"He worked hard over the summer and I now think I have a player there who has had an England cap and, over the piece, I think could force his way back into that squad if he keeps his head down and keeps working and playing like that against top, top players."
Mackay also praised the raucous support his side received from the 27,000-strong crowd as they claimed one of the most famous wins in the club's history.
"When the goals went in it was something quite incredible," he said.
"The stadium, the team, the staff and the fans did us proud in front of a worldwide audience.
"The passion the people have got for this club showed when the goals went in.
"It is something that is going to be massively important for us going forward. They are our 12th man."