She was the navigator aboard the commercial towing vehicle USCSS Nostromo. She was serving on the ship during its fateful voyage in 2122, when it encountered a Xenomorph on LV-426, an incident that eventually led to the destruction of the ship and the loss of all but one of its crew.
When the Nostromo set down on LV-426, Lambert was one of the three crewmembers chosen to investigate the signal coming from the moon's surface, along with Captain Dallas and Executive Officer Kane. The trio eventually stumbled across a derelict ship and, despite Lambert's discomfort and desire to leave, the decision was made to explore further. Inside Lambert and the others discovered the remains of the Pilot before Kane was attacked by a Facehugger in the ship's hold. Lambert and Dallas managed to carry Kane back to the Nostromo, where Ash let them in, despite Ripley's misgivings.
With each successive crewmember killed aboard the Nostromo by the Alien, Lambert became increasingly volatile, finally snapping after Dallas was taken inside the air shafts and hysterically insisting that the remaining survivors evacuate aboard the Narcissus, irrespective of the fact the shuttle would not take four passengers for any length of time. Although Ripley initially convinced her that their best chances lay with killing the Alien, once Ash — revealed to be an android sleeper agent — told them of Special Order 937 and pointed out the futility of fighting the creature, the three remaining survivors agreed to destroy the ship and make their escape after disconnecting Ash.
She was the last one to be killed by the Xenomorph. When she and Parker were collecting a large amount of extra coolant for the shuttle's life-support system if all three were to survive, she was killed by it after it killed Parker.