Lambert was the navigator of the commercial towing vessel Nostromo.
Lambert was one of the three chosen to explore LV-426 and who would eventually stumble across the Derelict. The ship made her uncomfortable and she wanted to leave. The discovered the remains of the Space Jockey, and Lambert wondered aloud what happened to the other members of the crew. After Kane was attacked by a facehugger, she and Dallas carried him back to the Nostromo, where Ash let them in, despite Ripley's misgivings.
She expressed a wish to leave the ship when crew members started getting killed off by the Alien. After Captain Dallas's death, she started to break down and started sobbing. When Science Officer Ash turned out to be an android, Lambert stabbed him with the crew's improvised cattle prod to stop him attacking them. This meant they had few enough people to escape on the shuttle safely, as it could only hold a maximum of three (Ripley, Parker and Lambert). However, they needed to get coolant for the life-support system, so Lambert and Parker went to get some while Ripley primed the ship's self-destruct sequence to kill the Alien after they left.
However, while gathering coolant, the Alien snuck up on Lambert, who was frozen with terror and could not flee, despite Parker shouting at her. Parker could not use his flamethrower for fear of injuring Lambert as well, so he charged at it. The Alien killed him though and then turned its attention to Lambert, who was sobbing pitifully. It seems that she was taken away by the Alien to be a host but she couldn't fit through a duct in which case the Alien "made" her fit. Ripley heard Lambert's screams and the Alien's roars over the ship's intercom and rushed to save her friends, but arrived only to find their corpses. Lambert's bloodied leg was seen hanging down, although the rest of her body as not shown.