Williamson first turned to writing and performing in plays in 1967 with La Mama Theatre Company and The Pram Factory, and rose to prominence in the early 1970s, with works such as "Don's Party" (later turned into a 1976 film), a comic drama set during the 1969 federal election; and "The Removalists" (1971). He also collaborated on the screenplays for "Gallipoli" (1981) and "The Year of Living Dangerously" (1982). Williamson's work as a playwright focuses on themes of politics, loyalty and family in contemporary urban Australia, particularly in two of its major cities, Melbourne and Sydney.