The plane was traveling from a hangar to a gate at O'Hare with it collided with the truck around 5 a.m., said Chicago Fire Department Chief Joe Roccasalva.
The driver of the truck was extricated from the city vehicle and transported in critical condition to Advocate Lutheran General Hospital in Park Ridge. Two mechanics from the plane were transported in good condition to Resurrection Medical Center in Chicago, Roccasalva said.
A runway at O'Hare was closed for less than two hours, according to Chicago Department of Aviation spokeswoman Karen Pride. Flight operations were not affected, she added.
SkyWest spokeswoman Nicole Drew said investigators reviewed United surveillance footage of the incident on Saturday, but due to the morning darkness and lights on the runway, officials "could not find who was responsible for the accident."
Chicago-based United Airlines planned to work with SkyWest Airlines, based in St. George, Utah., to determine what caused the collision.
"We will conduct a full investigation," said United Airlines spokeswoman Megan McCarthy.
The Federal Aviation Administration and the National Transportation Safety Board were also investigating the incident, Drew said.