A Long Island, N.Y., attorney yesterday admitted to stealing hundreds of thousands of dollars from clients who expected his help in adoption proceedings but argued that mental illness had driven his actions and that he should not be held criminally responsible.

But the prosecution rejected Kevin Cohen’s defense, saying in its opening statement at his trial that the Roslyn, N.Y., lawyer was a calculating, manipulative attorney who preyed on clients’ emotions to steal because he was living beyond his means.