Whittingham signed a new three-year contract with Cardiff in August despite advances from Premier League clubs
"I think he's the best player outside the Premiership," Mackay said.
"For him to commit to Cardiff City shows his ambition for the club and the team and the fact that sometimes you're at a club and you know you can still improve."He's got people here that will keep improving him and that have trust in him. The trust element's important between him and myself.
"Peter's been exceptional for me since I've been the manager and that's why I've got him to sign a three-year contract."
Cardiff have lost in the Championship play-offs for the past three seasons, but Whittingham has been particularly confident that the newly strengthened Bluebirds squad has the ability and depth to finally win promotion to the top flight.
"I think there's a bit of trust there between the staff, myself and him in terms of pushing him and wanting the best for him and him wanting the best for the team as well," Mackay added.
"He's somewhere where he feels comfortable, he feels as if he's leading group and I will keep pushing him to make him a better player.
"What we're seeing is an all-round player now and long may that continue."
Cardiff's win at Millwall, their first away victory of the campaign, kept them fifth in the Championship table although leaders Blackburn are just a point ahead.
The Bluebirds now travel to Crystal Palace on Saturday seeking to double their success on the road.