In 2004, fan-based organizations supporting the Seattle Sounders, Portland Timbers and Vancouver Whitecaps sponsored the creation of a 2-foot-tall silver cup to be awarded annually to the club that finishes with the best record in the season series between the three rivals. From 2004 to 2006, the Cascadia Cup competition included all scheduled regular season A-League and later USL-1 matches played between the Sounders, Timbers, and Whitecaps.
In 2007, the Virginia Beach Mariners folded just before the season started, leaving the USL 1st Division to scramble to fix the season schedule. The result was an unbalanced schedule, and as such it was decided to adjust the Cascadia Cup format for that season. Fans of all three Cascadian teams agreed to make only the last home and last away matches between constituent clubs count toward the Cup.
In 2008 the clubs reverted to playing each other an equal three times each. The Cascadia Cup competition once again included all scheduled regular season USL-1 matches played between the three rivals.
The Seattle Sounders of the USL-1 were made defunct in 2009 due to the introduction of the MLS expansion franchise of the same name. In 2009, it was announced that expansion teams would be allotted to Vancouver and Portland. Both teams were named after their predecessors, ensuring the continuation of the rivalry. Supporters of the Timbers and Whitecaps decided to continue the Cascadia Cup without the Sounders for two years while playing in the USL. In 2009, supporters of the Timbers and Whitecaps only included regular season USL-1 matches played between the teams and Portland won the cup for the first time. Some Sounders supporters wished to include a preseason victory over Vancouver and a US Open Cup match win at Portland for consideration in the cup standings while others believed that the Cascadia Cup competition should be suspended until 2011. Portland won the Cup for a second consecutive year in 2010.
The 2011 MLS season marked the first season since 2008 in which all three teams would be in the same league. Seattle claimed their third title by going undefeated in Cascadia Cup play – coming from behind to win both of their road games in Portland and Vancouver while earning draws at home.
For the 2012 MLS season the league changed to an unbalanced schedule. As a result, each team will play the other two Cascadia club three times each. Portland had four games at home while Vancouver and Seattle had three and two, respectively. It was decided by the supporters groups to count all matches equally. Portland won the cup in Vancouver on October 21, 2012, with their only away win of their 2012 season. It was the Timbers' third time hoisting the cup, and the first with the Sounders in the competition.
In December 2012, MLS filed a trademark claim for rights to the name "Cascadia Cup". This caused an immediate reaction from the three main supporters groups that had created the original competition and prompted them to form a legal entity called the Cascadia Cup Council, whose sole purpose was to obtain and retain the legal rights to the name "Cascadia Cup". Releases by MLS indicate that their trademark plan was an altruistic attempt to protect the Cascadia Cup from abuse by third parties, a claim that was disputed by the contest-organizing fan groups. Those same groups later had discussions with MLS leadership in an attempt to resolve the issue, and in July 2013 both MLS and the supporters groups announced an agreement that the Council would own the trademark and no monetization would occur without the agreement of all parties.
As in 2014–2016, the 2017 Major League Soccer season contains nine scheduled Cascadia Cup matches: each team plays both others three times in an unbalanced schedule. Each team hosts three of their six Cascadia Cup matches at home. The Portland–Seattle series has two matches in Seattle and one in Portland; the Portland–Vancouver series has two matches in Portland and one in Vancouver; and the Seattle–Vancouver series has two matches in Vancouver and one in Seattle.