In the last year and a half, about two-thirds of staff members of Maine’s most prominent mental health advocacy organization left the agency because its leader fostered a “toxic” work environment, according to interviews with 15 former employees and an additional employee’s resignation letter. Jenna Mehnert, the chief executive officer of NAMI Maine, which runs a network of resources to support people with mental illnesses and raises awareness of mental health issues, handed down unpredictable reprimands, acted combative when people asked her questions, criticized employees behind their backs and spoke down to them directly, making it difficult for people to succeed in their jobs, said the former employees, many of whom quit to preserve their own mental health. At least two former employees complained to the nonprofit’s board of directors in the last eight months to share their concerns about how Mehnert treats staff.