Construction of the four-station, 5.5-mile (8.9 km) branch line began in 1999 and was completed in under two years due to the use of local and private financing and existing public right-of-way.

In 1975, Multnomah County leaders negotiated reducing the number of car lanes along a future nine-mile (14 km) section of the controversial Interstate 205 (I-205) freeway.

[17] As a compromise, Metro published a transit plan in 1989 that reasserted the Westside Corridor's priority and commissioned preliminary work for the I-205 proposal.

[18][19] Ensuing alternatives analyses eventually caused Metro planners to shift light rail plans away from the I-205 corridor in favor of another north–south route farther west closer to downtown Portland;[20] this route, between Hazel Dell, Washington and Clackamas Town Center, became known as the "South/North Corridor".

[31] After long deliberations, agreements were made between Bechtel, the Port, TriMet, and local governments and agencies in October 1998.

[34] The following month, the Associated Builders and Contractors filed a petition in Multnomah County Circuit Court claiming that the contract awarded to Bechtel may have violated Oregon procurement laws.

[45] Much of the Airport MAX extension used public right-of-way already owned either by the Oregon Department of Transportation, the Port, or TriMet.

This avoided displacing private property owners and limited the project's impact only to parking spaces at Gateway Transit Center and along Airport Way.

[46] To minimize lane closures, workers used a cast-in-place concrete pouring method to extend the bridges' spans in 16-foot (4.9 m) increments.

[57] Upon opening, the Red Line operated from the airport to the Library and Galleria stations in downtown Portland, where its trains turned around at the 11th Avenue loop tracks.

[55] Although tested during trial runs,[48] TriMet opted to omit luggage racks from Red Line trains to maximize rider capacity.

At the time, service had been using single light rail cars,[61] but the influx of riders prompted TriMet to temporarily deploy two-car consists, which it had not planned to do until 2006.

[62] On September 1, 2003, TriMet extended Red Line service farther west via the Westside MAX segment to Beaverton Transit Center.

This was done in an effort to increase capacity between Gateway Transit Center and Beaverton and to provide a one-seat ride to the airport for the west side.

[64] In March 2008, three trips in each direction during the morning and evening rush hours began operating between the Hatfield Government Center and Portland Airport stations to provide further capacity on the Blue Line.

Additionally, TriMet had announced it would procure up to eight new light rail vehicles to accommodate the improvements,[67] but later purchased 30 new trains overall; four were part of A Better Red, while the remaining 26 were replacements for the original MAX fleet, which are gradually being retired.

[70] Preliminary design work began in February 2018,[71] and TriMet adopted a locally preferred alternative in April 2019.

[78] From April 2–9, 2022,[79][80] Red Line service was suspended to make way for construction, and shuttle buses operated between Gateway Transit Center and Portland International Airport.

[85][86] From January 14 to March 3, 2024, TriMet suspended MAX Red, Blue and Green Line service between NE 7th and Gateway Transit Center.

[92] The opposite end of the Red Line continues to be shown simply as "Airport" (referring to Portland International) on the trains' destination signs.

[2] One year after opening, the line averaged 2,800 daily riders at the airport, ahead of TriMet's first-year projections of 2,300.

[114] IKEA's opening in July 2007 helped attract more riders to Cascade Station shopping center, which had been considered a failed planned development following the economic recession caused by the September 11 attacks.

[115] In 2008, Cascades MAX station recorded an eight-fold increase in traffic, from 250 passengers per week to 2,000,[116] and this figure had further grown to 6,000 by 2010.

[117] The Red Line's annual ridership peaked at just over nine million passengers in 2009,[118] and it continued to fall over the next decade as part of a system-wide decline attributed to crime and rising housing costs in the Portland area.