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.
Source: PA
Source: PA