The area of present-day Washington State has been home to many Native tribes for some 12,000 years. Non-native explorers and traders started coming in the late 1700s; Lewis and Clark passed through in 1805. The United States and Great Britain both claimed the territory from 1818 to 1846. (See About Washington, Historylink.)
In 1848, the United States created the Oregon Territory, including present-day Oregon, Idaho, Washington State, as well as part of present-day Montana. Congress created Washington Territory in 1853. Act of March 2, 1853 (Organic Act), ch. 90, 10 Stat. 172 (1853).
In 1878, Washington Territory legislators held a constitutional convention in Walla Walla. That November, the voters approved the constitution. But Congress never took action on the bill required to make Washington a state. Elizabeth Gibson, First Washington Constitutional Convention convenes in Walla Walla on June 11, 1878, Historylink.
In 1889, Congress passed an act to enable the people of North and South Dakota, Montana, and Washington to form constitutions. Enabling Act, ch. 180, 25 Stat. 676 (1889).
A constitutional convention met in Olympia from July 4 through August 22, 1889, and drafted a constitution. The voters approved it on Oct. 1, 1889. President Harrison issued a proclamation admitting Washington to the Union on November 11, 1889. 26 Stat. Proclamation, p.10. [HeinOnline]