There were 250,500 people in its service area in 2018,[1] which spans the Tri-Cities metropolitan area except for Richland, which retains a city library: West Richland, Kennewick, and Pasco, and surrounding Benton, Franklin, and parts of Adams Counties are part of Mid-Columbia.

It was founded in 1948, originally to serve patrons in unincorporated parts of the counties.

[2] As of 2016[update], the last city to be annexed was Kennewick, and several cities which were still independent on paper, including Pasco and West Richland, paid Mid-Columbia to gain patronage rights for city residents.

[3] The system considered expanding to cover Walla Walla County Rural Library District in 2012.

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