The car had been driven 3km down the farmer's race in Carterhope Rd.
The farmer alerted police, and search and rescue crews from Owaka, Balclutha and Milton were called to look for Mr Mathieson, Sgt Bull said.
Mr Mathieson was diabetic, Sgt Bull said.
Police believed he became lost while driving home.
"He's got a condition that indicates to us certain patterns of behaviour."
Mr Mathieson was seen about 8.30pm on Wednesday in Mill Rd off State Highway 1 between Clinton and Balclutha and was reported to be disoriented, Sgt Bull said.
He was seen again, further west, on nearby Moa Hill Rd, where he stopped briefly.
"It was a man he knew and they had a good talk for a while," Sgt Bull said.
"There was no indication that he was lost when he was talking to that person, but we believe he was lost."