Peggy Ray of Everett said she was taking the money to pay her landlord Saturday afternoon when she saw a car, driven by a 74-year-old man and carrying a 68-year-old woman, strike a pickup truck and run off the highway, KIRO-TV, Seattle, reported Tuesday.
"I was just thinking, 'Oh my God. The car is smoking. We need to get these people out,'" Ray said.
She said she left the passenger door of her car open when she and another witness went to break a window and free the couple from the car, which was stuck in a water-filled ditch.
The Good Samaritan said she returned to her car after medics arrived and discovered her envelope filled with $900 in cash had been stolen from her car.
Police said more than 20 people stopped at the scene of the collision. The theft was under investigation.
Authorities said the man and woman from the car are expected to recover.