Ammond Troyer, 67, was driving the buggy west on Lattimer Road at 8:54 a.m. Wednesday when he failed to yield at the stop sign at the Ohio 13 intersection, leading to the collision with the southbound semi, said troopers with the Richland County of the state Highway Patrol.
After the collision, the buggy traveled off the left side of the road and ejected its three occupants, troopers said.
Troyer was transported to the OhioHealth Mansfield Hospital, where he was listed in critical condition while being treated for his injuries.
Two juvenile occupants in the buggy sustained non-life-threatening injuries and also were transported the Mansfield hospital, troopers said.
The truck driver, Bradley Weiser, 53, of Wooster, was not injured, troopers said.
The crash remains under investigation.