A point proved enough for Steve Bruce's side, but they had to endure an agonising wait as Watford - their only rivals for the second and final automatic promotion place - succumbed to a late 2-1 home defeat by Leeds United in a match delayed by around 15 minutes.
Hull thought they had done enough when Paul McShane's 63rd-minute goal put them 2-1 ahead at the KC Stadium.
But two penalties in stoppage-time gave the afternoon unexpected twists.
First Nick Proschwitz missed his chance from the spot to give Hull a 3-1 lead before Nicky Maynard, a minute later, scored from 12 yards to make it 2-2 after Abdoulaye Faye's handball.
Cardiff, who had already earned promotion to the Premier League as champions, finished the (second tier) Championship season on 87 points with Hull second on 79.
Watford ended third on 77 and will go into the playoffs.