Loic Remy scored twice in a dominant first-half display to put Newcastle in firm control as Cardiff found themselves on the back foot in the opening 45 minutes.
Cardiff improved in the second half and pulled a goal back through Peter Odemwingie, but it was not enough for the home side as Newcastle held on to take all three points.
Mackay admitted his side showed too much respect to Newcastle in the first half and that their second-half showing was too late to rescue the game.
"You cannot start the way we did and come away with something, we nearly did in terms of our second-half performance but you can't allow good players time and space to go and get two goals," Mackay told Sky Sports.
Asked if his side's first-half performance was uncharacteristic of his side so far this season, Mackay added: "Very much so in terms of how we play.
"Certainly the second half and first half were completely different. We allowed them too much time and space and got punished.
"It is frustrating as we were excellent second half because we were back to what we are and on another day could have got something back, could have got a draw out it."