Mazzarri's men - who have lost just one Serie A fixture in the current campaign - took the lead in the 18th minute thanks to Fredy Guarin's far-post effort.
But Sampdoria were well worth a point and got exactly that when substitute Renan Garcia fired an exceptional 25-yard strike past Inter goalkeeper Samir Handanovic in the 89th minute to finish the tie at 1-1.
Inter missed a chance to go third in Serie A and Mazzarri was unhappy with his side's effort, admitting his former club deserved a draw.
"In terms of the way I see football, it's the worst game my team has played this season," he said. "We've always taken the game to our opponents, attacked them, like at Bologna for example. Today though we were a bit off-color.
Despite his team's poor showing in his eyes, Mazzarri also credited the opposition.
"Samp played very well against us and deserved a draw. We had a number of chances, we just needed to be meaner and more clinical, more experienced, just come up with some extra to put the game to bed. I didn't like the intensity we showed both with and without the ball," he said. "Obviously well done to Samp, but today's match was the worst we've played since I've been in charge, at least in terms of our football. A draw is a fair result."
Mazzarri was disappointed his side could not win for new president Erick Thohir, who bought a 70 percent stake in the club in October and was in the San Siro stands for the first time Sunday.
"It's just a shame we couldn't give him a win, it would have been great it to the new president and also good in terms of the league table, because we've dropped two more points after those we let slip against Torino, Bologna and Cagliari," he said. "Anyway, the president clapped us at the end. That's a great sign. But it hurts more to throw away points like this.
"We hope the president stays cool and calm as he is now. Then, of course, to have a good league season you need investment and good organization."