Whittingham converted with force at the far post from Craig Noone's precision cross in the 65th minute to give Cardiff a deserved three points which gave them some valuable breathing space over the bottom three.
The 29-year-old has been a Bluebirds favourite for some time and delivered at a crucial time for Cardiff as they elevated themselves into 14th in the top-flight standings.
Whittingham 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," Mackay told reporters.
"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."
Although Whittingham is not renowned for headed goals, Mackay spoke of his memories of being on the wrong end of the playmaker's 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'."
After a run of five games without a win, Cardiff entered the clash teetering on the brink of the relegation zone but have started to distance themselves with three points.
The hosts enjoyed a lively start in the South Wales driving rain, winning numerous set-pieces in the opening exchanges, and Mackay believes his side were deserved winners on the day.
"I thought we started really well and put West Brom on the back foot right away. It gave us that momentum," he added.
"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."
Mackay will now take his men to Anfield to face in-form Liverpool next Saturday, before a Boxing Day date with Southampton back at the Cardiff City Stadium.