It happened Sunday night in Fort Worth. Hotel management told KXAS-TV the man demanded the facility's guest list before lighting the fire.
The clerk suffered minor injuries and was expected to recover.
The suspect, described as a white man in a teal T-shirt, took off in a white 4-door car, believed to be a Honda Accord with a sunroof and rear spoiler.
A Fort Worth Fire Department spokesman called the incident "absolutely horrific" on the agency's Facebook page.