The SUV he was driving went down an embankment an hour southwest of Madison, Wisconsin. The vehicle hit two trees, killing the 27-year-old, the Iowa County Sheriff's Department said.
What led Mentell's Jeep to veer off the road is still unknown. The crash was reported Monday.
The sheriff's department says the 27-year-old Mentell wasn't wearing a seat belt and that it did not seem drugs or alcohol were a factor.
Mentell portrayed attorney Garrett Wells on "Boston Legal" from 2005 to 2006.
According to the film database IMDb.com, he also was Terrell in the 2009 Disney movie "G-Force."
The Web site says Mentell was a former member of the U.S. Junior National Speed skating team and that he placed third in the Junior Olympics.
A message left with his agent wasn't immediately returned Tuesday.