Manning studied law at Lincoln's Inn. In 1844 he was appointed as Solicitor General of the colony. He serve in the Legislative Council from 1851 until its disolution when he was elected to the Legislative Assembly. He was later appointed Attorney General and then Queen's Counsel. On the reconstitution of the Legislative Council in 1861 he was elected a life member but resigned in 1876 to become Judge of the Supreme Court, having originally been offered the position in 1859. He was knighted in 1858. He retired as judge in 1887 to rejoin the Legislative Council. In 1878 he was elected Chancellor of Sydney University.