Flight 1603 was diverted to Boise Airport in Idaho, where it made an emergency landing around 8 p.m., Boise Airport spokeswoman Patti Miller confirmed.
"We got a man down, chest compressions going on right now. I'm not sure too much right now the status," A United crew member said in a recorded conversation with air traffic control. "Can an ambulance and maybe some air stairs meet us on the runway?"
A statement from United Airlines confirmed paramedics met the plane when it landed in Boise and transported the captain the Saint Alphonsus hospital.
"I am sad to confirm that our co-worker passed away last night," a United spokesman said Friday. "Our thoughts are with his family at this time."
A passenger aboard the flight said a crew member asked on the loudspeaker if anyone aboard was a physician.
One woman, a first-year resident doctor, volunteered to help perform CPR on the captain, according to passenger Ken Martin, who was sitting next to her. She described the as a large man, weighing more than 300 pounds, and said he had to be moved out of the cockpit and into the first class cabin to perform CPR.
Martin said she told him they were unable to revive the pilot and was "visibly upset" when she returned to her seat.
Six crew and 161 passengers stayed in Boise for several hours until United was able to find another captain to continue onto Seattle.
Couple mistakenly served bag of cash at McDonald's drive-thru
Exploding whale video goes viral on Internet