William G. “Bill” Robertson plans to leave his post as president and chief executive of Gaithersburg-based health system Adventist HealthCare in April to lead MultiCare Health System, a not-for-profit organization based in Washington state.

Robertson has led Adventist — one of the largest private employers in Maryland, with more than 6,200 employees — since 2000. His last day will be April 4, Adventist HealthCare Board of Trustees Chairman David E. Weigley announced Monday.

Robertson is slated to become the president and chief executive at MultiCare Health on May 1.

MultiCare is based in Tacoma and has about 10,000 employees and five hospitals.

Under Robertson’s watch, Adventist became the largest provider of health care in Montgomery County and added several new facilities including Adventist Rehabilitation Hospital of Maryland, Adventist Behavioral Health, the Reginald S. Lourie Center for Infants and Young Children, Adventist Medical Group and the Shady Grove Adventist Hospital Aquilino Center.

Expansion efforts ran into trouble in 2011, when Adventist failed to win approval for plans to build a comprehensive medical campus and hospital in Clarksburg.

The system has since agreed to sell the 47-acre site to a developer who proposes a retail outlet center.

“The organization has a very bright future and is well-positioned to be a leader in the state as more of an emphasis is placed on preventive health and wellness,” Robertson said in a statement.

Adventist has yet to select a replacement, Weigley said in the statement. The health system plans to release details of the succession process in the new few weeks, an Adventist spokesman said.

“I take great comfort in knowing that Bill built an excellent team of executives and leaders who will help me with the selection process for a new president and CEO,” Weigley said.