The risk of violence increases the longer psychosis goes untreated, Insel added. He argued for "closing that gap" between the onset of symptoms, diagnosis and treatment.
Mental-health issues are receiving attention in the wake of several deadly shootings allegedly perpetrated by young men with severe mental and emotional problems.
President Obama's plan to curb gun violence included several measures related to mental health, like a commitment to finalize mental health parity rules, and Congress has seen a string of new bills to increase individuals' access to treatment.
At a Senate Health, Education, Labor and Pensions (HELP) Committee hearing on Thursday, where Insel spoke, lawmakers urged the public to remember that mental illness is not inherently linked with violence.
"There is still a stigma associated with mental illness," said HELP Chairman Tom Harkin (D-Iowa), "and that stigma results in too many people suffering in silence without access to care."
One in five people in the United States suffers from mental illness, and one in 20 experiences more serious psychotic disorders, which might include paranoia, hallucinations and hearing voices, Insel said.
"Most violence has nothing to do with mental illness, and most people with mental illness are not violent," he told the panel.