Arthur C. Evans Jr., the commissioner of Philadelphia's Department of Behavioral Health and Intellectual disAbility Services, is leaving the city after 12 years to become CEO of the American Psychological Association, the organization announced Wednesday.

He will begin work at the APA, which has 117,500 members, on March 20.

In a statement, APA president Antonio Puente said Evans was a "trailblazer who has relied on the strength of his background in both research and clinical practice to create transformational change in each of the mental health systems he has led."

Evans, 57, said in an interview that his work in Philadelphia has been "really hard and challenging," but that he also had become very comfortable here.  "For me, it's not good to be comfortable," he said.  "I think it's important for me to continue to do new things and to continue to challenge myself."

He called the new job a "rare opportunity." He will be only the 11th CEO of the association in 125 years.

Evans said he will work in Philadelphia until Feb. 10 and will continue with a task force on opioids into March.  His deputy, David T. Jones, will be acting commissioner.  Evans said he hoped the appointment becomes permanent.

Jones came to Philadelphia 3 1/2 years ago after serving as chief of behavioral health and crisis services with the Montgomery County (Md.) Department of Health and Human Services.

Mayor Kenney said in a statement that Evans had "spearheaded a systemwide transformation" of the behavioral health department, "de-stigmatizing mental health issues and making its services more accessible for our most vulnerable citizens."

Evans said he was proudest of orienting Philadelphia's services toward a recovery and resilience model, one that assumes that people with serious mental illnesses and disabilities can have meaningful lives in the community.

"We have fundamentally changed how people think about care and approach service delivery," he said.  "Where most people still are is a model of care that focuses on people's symptoms."

Evans said he also hoped to bring his emphasis on evidence-based treatment to his new job. He will succeed Norman Anderson, who resigned at the end of 2015.  Cynthia Belar has been interim CEO since then. The APA has a $128 million budget and more than 500 employees.

In Philadelphia, Evans has led a department with a $1.2 billion budget that provides services for 1.5 million Philadelphians.

Before coming to Philadelphia, he was deputy commissioner of the Connecticut Department of Mental Health and Addiction Services.

Evans has taught at the Yale University and University of Pennsylvania medical schools.  He has a doctorate in clinical/community psychology from the University of Maryland and a master's in experimental psychology from Florida Atlantic University, where he was an undergraduate.

Evans said he knew no one here when he came to Philadelphia during the administration of Mayor John F. Street.  He has come to appreciate, he said,  that Philadelphia is a place that "isn't afraid to be first" or to "push the envelope on behalf of people."