William was educated at country state schools, including Kiama, and the University of Sydney, where he won a scholarship.

[9] Cullen entered politics in 1891 when he was elected a member of the New South Wales Legislative Assembly for Camden.

[11][12] Though not a strong party man, or even a politician by temperament, he was a useful member of the house who never spoke unless he could contribute something constructive to the debate.

During his early days in the Legislative Council he introduced a bill embodying important reforms in the conduct of the university, although some of these were not brought into effect until many years after.

[2] Cullen found much business awaiting him at the Supreme Court, but his great capacity for work soon cleared up the arrears.

He was a very sound equity and constitutional lawyer who as chief justice worthily upheld the traditions of his court.

Cullen was interested in literature, in the Australian flora (Eucalyptus cullenii was named for him), and in social and philanthropic movements.