The Seagulls put a dampener on Mackay's 41st birthday celebrations as goals from former Swansea midfielder Andrea Orlandi and Leonardo Ulloa condemned Cardiff to just their second home defeat of the season.
David Lopez set up both goals, putting a perfect ball in for Orlandi to open the scoring two minutes prior to half-time, before pulling the ball back for Ulloa to grab his first league goal for the club with a deflected strike.
But Mackay will have been left wondering how his side failed to make the most of a host of chances as they were denied four times by the woodwork and by a superb performance from Brighton keeper Tomasz Kuszczak.
The Scot was also less than impressed by referee Keith Stroud's decision not to point to the spot with his side 1-0 down close to the hour mark.
Craig Bellamy appeared to be brought down in the box by Brighton defender Adam El-Abd, who had already been booked, but the official awarded a free-kick on the edge of the area.
He said: "I spoke to the fourth official about it and asked him the ruling.
"Their player was inside the area and Craig Bellamy was on the line on his way in and the ruling is that is a penalty.
"On the line is inside so unfortunately the referee got it wrong. I thought he had a good game but it was a key decision and he got it wrong."