Whittingham converted superbly at the far post from Craig Noone's cross in the 65th minute to give Cardiff a deserved three points which gave them some valuable breathing space over the bottom three.
Whittingham has been a fans' favourite at the Welsh club for many years, but found himself out of the side during the latter stages of the Championship title success.
However, he has responded superbly this season and Mackay hailed the efforts he has made in adapting to his return to the Premier League.
"I'm delighted for him because he's been one of my most consistent performers this year," said Mackay.
"He had a period out of the side near the end of last season but really came back in the summer with a renewed belief in his own ability.
"He worked hard on his body to survive in the Premier League in midfield and he's showing that this season."
The method of Whittingham's goal may have been unfamiliar but Mackay revealed he had been on the receiving end the last time the playmaker scored with his head.
"Peter scored a headed goal against me when I was the manager of Watford about four years ago," he said.
"I told him that. I said, 'do you remember the last time you scored a header?' and he went, 'against you at Watford'."
Mackay was also pleased with the overall performance of his team as they ended a run of five games without a win.
"I thought we started really well and put West Brom on the back foot right away. It gave us that momentum," he said.
"I knew at half-time we had to be patient and eventually it would come.
"I'm delighted for the players. It's an important win for us against a team who are around us in the league."
Defeat means West Brom have suffered four straight defeats and have only won one of their last 10. They are just two points above the relegation zone.
The Baggies created very little until forced to chase the game - their best chances saw Gareth McAuley denied by Gary Medel's clearance on the line and David Marshall save superbly from Shane Long's header with four minutes to go.
Long also had a penalty appeal turned down after Ben Turner appeared to bundle him over in the box.
"We created some chances as we normally do," said manager Steve Clarke. "At the moment things aren't quite going for us.
"I don't think it's a lack of confidence. We tried to pass the ball but Cardiff started brightly and put us on the back foot.
"We resisted them and stayed in the game. The longer the game went on, the more we played and the more chances we created. It's difficult."
On the penalty incident, he added: "Shane said in the dressing room that he thought it was a penalty.
"Unfortunately, Howard Webb didn't think it was a penalty.
"After the game, what else can I do? I can rant and rave and criticise referees but it's not my style.
"We have to work harder and hope we get a lucky break or that somebody does something fantastic. There is no magic formula or wand.
"It's difficult for the players. All we can do is keep trying as hard as we can to break the run as quickly as possible, starting next week against Hull."