Lambo was taking her dog Barney for a walk when she was hit.
She was pronounced dead at the scene. Her dog was also killed.
Police say a passenger got out and untangled Lambo's walker from the fender before the truck sped off.
A male suspect was arrested Wednesday night. He has not yet been identified.
The hit-and-run death has been ruled an accident by the Los Angeles County Coroner's office.
Lambo was the widow of former Montebello City Clerk Andrew Lambo.
Anyone with information is asked to call Montebello police at 323-887-1313.