Walker was admitted to the New South Wales bar in 1979. He was appointed Senior Counsel in 1993, and was president of the NSW Bar Association from November 2001 to November 2003, having been vice-president from 1996 to 2001. He was president of the Law Council of Australia from 1997 to 1998.
Walker is a member of the Council of Law Reporting for New South Wales, and has been editor of the NSW Law Reports since 2006. He is a patron of the State Library of NSW as a foundation senior fellow and has been a member of the NSW Health Clinical Ethics Advisory Panel since 2003. He was governor of the Law Foundation of NSW from 1996 to 2007, and Special Commissioner of Inquiry for the NSW government into Sydney Ferries in 2007. He has been a director on the board of the Sydney Writers' Festival since 2000. He was a foundation member and has been director of the Australian Academy of Law since 2007.
In April 2011, Walker was appointed as the independent national security legislation monitor. In 2015, Walker suggested that a proposed change to Australia's citizenship laws to give the power to the immigration minister to strip citizenship of people who support terrorism was unconstitutional, this was a misquote of the INSLM 2014 report by prime minister Tony Abbott. In a 2015 interview with Lateline, Walker criticised the Australian "habit of seeing a problem and passing a law about it".
He charges $14,000 a day.
- Find a Barrister, NSW Bar Association
- Who's Who Australia – entry for Bret William Bret
- Bar News, Summer 2001/2002, p. 2.
- Bar News, Summer 2003/2004, p. 2.
- , Our Presidents, Law Council of Australia
- See glass placard at Macquarie St entrance to the State Library of NSW, or see also: 
- "Board". swf.org.au.
- "Australian Academy of Law - Home". academyoflaw.org.au.
- "About". The Red Room Company.
- "PM - Govt defends plain packaging in High Court 18/04/2012". abc.net.au.
- "Home - Fairfax Media Limited". canberratimes.com.au.
- "Finance & Political Breaking News Australia & Worldwide - afr.com". afr.com.
- Nathan Morrow. "Independent National Security Legislation Monitor appointed". altlj.org.
- "Bret Walker: Revoking citizenship by minister unconstitutional". NewsComAu.
- "Government misquoting my report to defend revoking citizenship, says Bret Walker". Guardian Australia.
- "Lateline - 13/10/2015: Interview: Lawyer Bret Walker SC, the first independent monitor of national security legislation". abc.net.au.
- Berkovic, Nicola (12 June 2014). "Top tax barristers pulling in $25,000 a day". Retrieved 10 July 2016.