Mamu was sworn in by Chief Justice Sir Salamo Injia on Friday in Port Moresby.
The office of the Public Solicitor provides legal aid service ranging from criminal to civil, including family law.
Since the elevation of the former Public Solicitor (now Justice) Jim Wala Tamate to the National and Supreme Court bench, Leslie Mamu, was the acting Public Solicitor.
He was appointed Public Solicitor on July 30 by the Judicial and Legal Services Commission.