Ryan Lacroix, 38, of Rhode Island was injured July 4 in a crash on Pond Road in Mount Vernon.
Lacroix was airlifted to a hospital but he died on Sunday.
The Kennebec County Sheriff's Office said Lacroix was thrown from the ATV he was riding and was found unresponsive in the roadway by a passerby. Deputies suspect speed, alcohol and driver inexperience may have been factors in the crash.