Police say Stewart was heading eastbound on Bear Mountain Bridge Road in Cortlandt just before 9 a.m. Wednesday when his SUV veered into the westbound lane and hit a rock wall.
No other vehicles were involved in the crash.
He was rushed to Westchester Medical Center, where he is said to be in stable condition.
An investigation is now underway into whether a medical emergency contributed to Stewart's accident.
Alexander was appointed for a period of 60 days.