Somsak Paelee, who was known widely as Kamnan Huan, was murdered around midnight, his driver told police on Tuesday.
Their pickup truck was chased and shot at by gunmen riding in another pickup truck.
Their bullets hit Somsak and his security guard, Srichol Phuengsuk. Both were taken to a nearby hospital, where Somsak later died.
Pol Lt Col Vatcharapol Phothirat, an inspector at Prachinburi police station, said Somsak went to a funeral rite at a temple and was on his way home when he was attacked.
The driver, Wanisara Pibul, told police that he realised there was another pickup truck tailing his car and later that truck sped up, two men in the back stood up and started shooting at his vehicle.
The guard, Srichol, who sat in the front of the vehicle, fired back with a handgun.Somsak was in the back seat.
The attackers then fled.
Police suspected the killing was linked to local political conflict around the election of the head of the Ta Toom tambon administration organisation.