It happened at about 12:30 p.m. Monday on Straight Path and Wyandanch Avenue in Wyandanch.
Police say the patient, Michelle Garrett of West Babylon, was taken to Good Samaritan Hospital in West Islip where she was pronounced dead.
Suffolk County Police said Tuesday that the ambulette's driver, 56-year-old Gerard Gruenthal of North Babylon, has also succumbed to his injuries.
The truck driver, 42-year-old Lynn Godwin, was airlifted to Stony Brook University Medical Center, where he was listed in serious condition.
The investigation is continuing.