Championship: Brighton 2 Leeds 2
The Scottish striker was also denied a hat-trick after Leeds goalkeeper Paddy Kenny saved a 74th-minute penalty, leaving Gus Poyet's men still searching for their first win in seven league games.
Mackail-Smith put the hosts ahead from the penalty spot on 16 minutes after midfielder Will Buckley was fouled by Leeds captain Lee Peltier.
Leeds started to find their rhythm following the goal and soon found themselves level when Spanish defender Bruno brought down Luke Varney inside the box.
El-Hadji Diouf, who kept Luciano Becchio on the bench for a second successive game, repaid his manager's faith with a successful penalty in the 37th minute, his second goal in as many games.
However, Brighton regained the lead just three minutes into the second half through Mackail-Smith.
Buckley squared a ball to Mackail-Smith and the 28-year-old made no mistake from close range.
Leeds refused to go away and they were rewarded for their persistence with a 66th-minute equaliser.
Former Wigan midfielder Michael Brown unleashed a curling effort, which found its way into the bottom corner courtesy of a deflection off substitute Becchio.
Brighton then had a chance to hit the front yet again but Kenny was up to the task, saving Mackail-Smith's spot-kick with less than 20 minutes remaining at Falmer Stadium.
The result sees Brighton move up to ninth, while Leeds climb one position into 10th.