"It's absolutely fantastic, I'm delighted for everyone at the football club," Mackay is quoted as saying by Sky Sports News.
"It was obviously a tremendous feeling on Tuesday night but for us to go and win the title is something that we then spoke about within a day. We wanted to go and get the boys their medal and win the championship.
"We've been sitting there since late November and it's been an awful long time for us to be top and be chased and be examined. I think they deserve it. I'm delighted that we've had the mental strength and character to have that consistency and just keep going, being relentless. That's what we were this year - relentless."
Cardiff's campaign in the Premier League next season will be their first ever.