In 1990, Mesa acquired Aspen Airways Denver hub and routes, except for Aspen's Denver to Aspen route.
It attempted to acquire Aspen's codeshare with United.
 However United was unwilling to codeshare with an airline that only operated 19-seat turboprops.
Mesa leased Embraer EMB 120 Brasilia aircraft from its former competitor in New Mexico, Air Midwest.
With the Brasilias in hand, Mesa gained a codeshare with United for its Denver hub.
Several Beech 1300 and Beech 1900 aircraft were repainted with the United Airlines scheme.

In 1995, California Pacific and its Los Angeles hub were merged into Mesa's United Express operation.
After the closing of Superior Airlines' Columbus hub, its aircraft and crews were used to expand United Express into Portland and Seattle.
In 1997, operational difficulties with the Denver hub and disagreements over the renewal of Mesa's WestAir subsidiary codeshare with United resulted in the cancellation of Mesa's codeshare.

In 2003, Mesa agreed to a service agreement with United for service out of their hubs at Chicago-O'Hare, Denver, and Washington-Dulles under the United Express banner.
In October 2009 United decided to exercise its early termination option for the Dash 8 flying.
The Dash 8 flying ended on April 30, 2010.
Around the same time, United decided not to extend its CRJ200 operation and as a result, all of the Mesa CRJ200s (26 aircraft) flying under United Express were phased out by April 30, 2010.
As of 2015, Mesa's United Express operations consisted of Embraer 175 flying out of the Houston hub and CRJ-700 flying out of the Washington Dulles hub.
In 1990, Mesa acquired Aspen Airways Denver hub and routes, except for Aspen's Denver to Aspen route. It attempted to acquire Aspen's codeshare with United.  However United was unwilling to codeshare with an airline that only operated 19-seat turboprops. Mesa leased Embraer EMB 120 Brasilia aircraft from its former competitor in New Mexico, Air Midwest. With the Brasilias in hand, Mesa gained a codeshare with United for its Denver hub. Several Beech 1300 and Beech 1900 aircraft were repainted with the United Airlines scheme.
In 1995, California Pacific and its Los Angeles hub were merged into Mesa's United Express operation. After the closing of Superior Airlines' Columbus hub, its aircraft and crews were used to expand United Express into Portland and Seattle. In 1997, operational difficulties with the Denver hub and disagreements over the renewal of Mesa's WestAir subsidiary codeshare with United resulted in the cancellation of Mesa's codeshare.
In 2003, Mesa agreed to a service agreement with United for service out of their hubs at Chicago-O'Hare, Denver, and Washington-Dulles under the United Express banner. In October 2009 United decided to exercise its early termination option for the Dash 8 flying. The Dash 8 flying ended on April 30, 2010. Around the same time, United decided not to extend its CRJ200 operation and as a result, all of the Mesa CRJ200s (26 aircraft) flying under United Express were phased out by April 30, 2010. As of 2015, Mesa's United Express operations consisted of Embraer 175 flying out of the Houston hub and CRJ-700 flying out of the Washington Dulles hub.
Mesa Airlines