The call about Flight 1651 came in around 12:10 p.m. and the plane landed safely at McCarran International Airport around 12:25 p.m., officials told ABC affiliate KTNV.
The cockpit door malfunctioned, locking the pilot out of the cockpit, airport officials told KTNV, noting that the first officer made the landing.
"About half way through [the 2.5-hour flight] there seemed to be some talking at the front of the plane. You could see the captain out there," passenger Jesse Dougherty told ABC News. "There wasn't a huge panic but some confusion."
The captain explained to the passengers that the door was jammed and he couldn't get back in, Dougherty said, adding: "It was very, very bizarre."
When the first officer made the landing "perfectly," the passengers broke out into spontaneous applause, Dougherty said.
Because the first officer was accustomed to the controls on the right seat of the cockpit, he remained there, the crew explained to passengers. That meant the only issue was a lack of taxiing controls once on the ground, necessitating a tow from the runway to the gate, Dougherty said.
No one was injured and there were 168 people on board the plane, officials said.
The source of the jammed door was a piece of string that was found near the door by the maintenance crew, passenger Jonathan Thalacker told ABC News.
The aircraft was an MD-90, airport spokeswoman Christine Crews told ABC News.
"There was a door malfunctioned that locked the captain out so the first officer had to do an unassisted landing," Crews said. "We take everything very seriously. This was an unusual landing. He called the airport so that we would have ground response available."