Tom Wagner told KTRK-TV he was supposed to change planes Friday at George Bush Intercontinental Airport. But no staff noticed he was still sleeping in a window seat at the rear of the flight from Lafayette, La., before they closed up the jet.
Wagner called his girlfriend who asked the airline to rescue him.
ExpressJet, which operates United Express in partnership with United Airlines, apologized Saturday and said the incident is under investigation. It says crew conducted a post-flight walk-through.
Wagner received a $250 flight voucher, was put up in a hotel and was flown to Orange County, Calif., on Saturday.