Leslie was a neighbour of Howard Stambler, who knew about his bunker. When the attack happened she went to take shelter on the bunker, but was caught by the toxic gas spread by the Alien Spaceships.
10 Cloverfield Lane
She arrived at the bunker, just as Michelle was going to open the door to escape. She begged her to let her in, but Howard order her to not do it. With a more aggressive tone Leslie demanded Michelle to let her in, and began to smash her head against the glass in a desperate effort to enter. Leslie died after that.
After Michelle escaped from the bunker, she was attacked by an Alien Ship and its scout, to which she hide on a small shed, where she found Leslie's corpse and took her car keys. After she destroyed the ship Michelle drove away on Leslie's car.
The Cloverfield Paradox
Leslie also makes a minor return in the newest sequel in which she interviews Mark Stambler.
Leslie dieing in the March of 2016 does not make sense, since she is alive in 2030 where The Cloverfield Paradox takes place. This may mean that 10 Cloverfield Lane takes place in a different dimension where Leslie died due to the aliens spraying a mist of chemical air and killing people.