Dr. Thomas R. Insel, the director of the National Institute of Mental Health, announced on Tuesday that he planned to step down in November, ending his 13-year tenure at the helm of the world’s leading funder of behavioral-health research to join Google Life Sciences, which seeks to develop technologies for early detection and treatment of health problems.

The announcement is no small personnel matter for the behavioral sciences.

Losing Dr. Insel leaves the agency — which is growing in importance and visibility in the wake of the Obama administration’s brain initiative — with a large hole, mental health experts in and out of government said. Dr. Insel has been an agreeable, determined, politically shrewd presence at an agency that has often taken fire from advocacy groups, politicians and scientists.

In hiring him, Google, which is in the process of reorganizing into a new company called Alphabet, lands a first-rate research scientist and administrator with an exhaustive knowledge of brain and behavioral sciences. He has also recruited some of the top researchers into the brain sciences from other fields.

“Tom’s leaving is a great loss for all of us,” said Dr. E. Fuller Torrey, the executive director of the Stanley Medical Research Institute, a nonprofit that supports research into severe mental illnesses. “He refocused N.I.M.H. on its primary mission — research on the causes and better treatments for individuals with serious mental illness.”