Owen Smith, 50, was convicted of five counts of rape, two of sexual assault and one count of trafficking after a trial at Sheffield Crown Court.
South Yorkshire Police said he abused his position to gain the trust of his victims.
Smith, of Hawley Street, Sheffield, was remanded in custody ahead of a sentencing hearing on 10 May.
The court heard he was arrested in September 2014 after being found with a teenage girl who had been reported missing the previous day.
Two further victims were identified as a result of a police investigation.
Det Con Stephen Evans said: "Smith preyed on vulnerable girls and women at risk of sexual exploitation, abusing his position as a mental health worker to gain access to their lives.
"He should have been someone the victims could trust, that the victims could turn to for help. Instead, he used them for his own sexual gratification, which is simply despicable."
A mental health worker has been found guilty of raping and assaulting three vulnerable females.