Hennepin County Library Director Lois Langer Thompson will leave her position in a few weeks to take over as director of a library system in Washington state.

Thompson, 57, who led the county’s libraries for 10 years, will begin as executive director of Sno-Isle Libraries in Washington in December. Her last day with Hennepin County will be Oct. 12.

She said Wednesday that it had been “an honor” to lead the Hennepin system, which has 41 libraries, more than 850,000 cardholders and an $86.2 million budget.

“I have absolutely loved being with Hennepin County,” she said. “I just felt like it was time for a change.”

Thompson said that county officials likely would select an interim director and conduct a nationwide search for her replacement.

“Whoever steps in as interim [director], I set it up well for them,” she said. “It’ll be a really smooth transition into the next director.”

Thompson was named interim director of the Hennepin County libraries in 2008, only months after the Minneapolis library system merged with the county’s. Thompson, who had worked for 20 years in the county’s libraries, was appointed library director in February 2009.

She said the merger was difficult work that allowed the county to serve a more diverse population. She said that she also was proud of the library system’s ongoing efforts to reach out to Somali, Hispanic and American Indian communities.

During Thompson’s tenure, the county built or expanded nine libraries and renovated 18 others. Together with the nonprofit Friends of the Hennepin County Library, the system increased its annual funding from $250,000 to $1.2 million.

“The library is very stable and just doing really great work,” she said. “It’s hard to leave a place that I have worked [for] so long, and I’m so proud of the work of the staff.”

In her new role, Thompson said she would help build support for the Sno-Isle Libraries, which are in Snohomish and Island counties on Puget Sound north of Seattle.

Kristi Pearson, executive director of the Friends organization, announced Thompson’s departure in an e-mail sent to donors Wednesday.

Pearson called Thompson a “one-of-a-kind, visionary leader” and praised her “strong leadership, trusted partnership, and most importantly, her wonderful friendship.”

Thompson will leave shortly after Ridgedale Library in Minnetonka, the system’s headquarters, reopens on Sept. 24 after a yearlong renovation.