William Harrison Moore was born in London, and after leaving school worked briefly as a journalist, before studying at the universities of London and Cambridge. In an era when professors were often much younger than today, he was appointed professor of law at the University of Melbourne at the age of 25, starting work there in 1893. 

At Melbourne, Moore specialised in constitutional law, which was transformed by Australian federation in 1901. He was the author of one of the first and most authoritative books on Australian federal constitutional law, The Constitution of the Commonwealth of Australia, and his teaching influenced generations of law students, among them future chief justice Owen Dixon and future prime minister Robert Menzies.

At a time when such a role was unusual for a professor, Moore became an adviser on constitutional issues to state and federal governments, and to the monarch's representatives, the Governor-General and the state Governor. He was the first of the Law School's professors to be knighted, becoming Sir Harrison in 1925.

Rising student numbers after World War I burdened Moore, as the only full-time member of the Law School's academic staff, and he retired early, in 1927. But he continued his work on international relations, and joined Australian delegations to the League of Nations and the 1929 London conference on dominion legislation.