Hillsborough County Sheriff Chad Chronister said during a Wednesday morning news conference that the woman and another adult living in the apartment made it safely outside when she noticed the children, ages 5 and 10, didn't make it out.
The sheriff said running back to save the children was a great "action of love" displayed by the mother.
When firefighters arrived, flames were already penetrating the roof, which later collapsed.
The names of the victims weren't immediately released.
Officials say at least 12 residents were displaced by the fire, which damaged eight apartments.
An investigation continues.
Copyright 2019 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.