Barker practiced as a Barrister and Solicitor in Alice Springs, Northern Territory from 1961 to 1970 and in Darwin from 1970 to 1974. In 1974, Barker became one of the first Queen's Counsel appointed in the Northern Territory and practiced at the Independent Bar at Darwin until his appointment as Solicitor General of the Northern Territory in 1978, the first such appointment after self-government was granted to the territory. He returned to private practice at the Sydney Bar in 1980. Barker became known nationally in 1982 when he led the prosecution in the Azaria Chamberlain trial and Lindy Chamberlain was tried and convicted of her murder and Michael Chamberlain was convicted as an accessory after the fact. Both were later exonerated and received substantial compensation
Aside from the Chamberlain trial, Barker QC has had many successes that put him on a footing as one of Australia's most successful barristers. For one, he successfully acted for John Marsden in a defamation case against channel seven where Marsden was wrongly portrayed as a paedophile. In a book written by Marsden before his passing; he referred to Barker QC as 'the best cross examiner in the land'. Barker QC also successfully defended ex Australian High Court judge Lionel Murphy for allegedly perverting the course of justice. Aside from these landmark cases, during his time in the Territory, Barker QC has made a great impact on the improvement of the indigenous inhabitants of Australia. He was successful in getting a bill through the Northern Territory Parliament making an area known as the Cobourg Peninsula a National Park so that it remained untouched by commercial development. Ian Barker QC was widely regarded throughout legal circles as one of the very best barristers in Australia.
Barker is a member of the International Human Rights Observer Panel which was established in 2005 by the Law Council of Australia. It is a part-time panel of Australian lawyers who serve as trial observers and undertake reviews in relation to human rights. He is regularly called upon by the national media as a commentator on matters relating to human rights and civil liberties.