Sewell studied science at the University of Sydney before embarking on a career as a playwright. While at university during the early 1970s he was involved in fringe theatre in King's Cross and had his first plays performed there in 1975.
He moved to Brisbane in 1975 where he worked as a scriptwriter for children's radio and became associated with the La Boîte Theatre Company which staged The Father We Loved on a Beach by the Sea in 1978. This play dramatises one family's interactions with the 'system', showing the division between generations and the destructiveness of each stance. Sewell is often described as a political writer, drawing inspiration from Catholicism and Marxism, but the author sees his work as an attempt to broadly represent the many interactions between powerful historical forces and powerless individuals. In addition to strong political themes, Sewell's plays offer actors and audiences a challenging Brechtian environment that often mixes naturalistic and expressionistic modes.
Sewell returned to Sydney in the 1980s after a brief period in Melbourne with the Australian Performing Group. His many plays are widely and regularly performed, and have attracted numerous awards. Sewell has also written scripts for television and cinema, and has been writer-in-residence at several theatre companies and institutions. In 2008-2009 Sewell was Literary Fellow at the University of New South Wales.