The Jaguar driver, 73, died in the incident while a man, 32, has been arrested on suspicion of offences including driving under the influence of alcohol. He was driving the Mercedes.
Police, firefighers and paramedics battled to save the fatally inured pensioner after the crash at just before 10.30am on Sunday.
A Met police statement said: “Police were called to reports of a collision involving a Jaguar and Mercedes on Hacton Lane in Hornchurch.
“Officers, the London Ambulance Service and London Fire Brigade attended. Despite the efforts of emergency services, the driver of the Jaguar, who is believed to be a 73-year-old man, died at the scene.
“His family have been informed and are being supported by officers.
“The driver of the Mercedes was taken to hospital as a precaution. The 32-year-old man has been arrested on suspicion of offences including driving under the influence of alcohol.”
Detectives from the Roads and Transport Policing Command are investigating. They would like to speak with anyone who witnessed the collision or the moments leading up to it.