The driver's passenger, who is also in his 70s, was also hurt in the collision between their Corsa and a LandRover Discovery.
The crash happened just south of the Cromarty Bridge, near Dingwall, at about 19:00 on Tuesday.
The female driver of the other vehicle involved was uninjured.
The driver of the Corsa was airlifted to Raigmore Hospital in Inverness.
The section of road was closed for a time to allow for the recovery of the vehicles and then again for three hours from midnight so police could investigate the cause of the collision.
This investigation was prompted by the deterioration in the 78-year-old man's condition. It was initially thought he had suffered non-life threatening injuries.
Police have appealed for witnesses.
A 78-year-old man is in a life-threatening condition in hospital after being seriously injured in crash on the A9, police have said.