Giles was born in Melbourne on 31 July 1973.^[3] His mother was born in the United Kingdom, making him a British citizen by descent; he renounced his dual citizenship before standing for parliament in 2013.^[4] Giles attended high school at Scotch College, Melbourne. Giles has a Bachelor of Arts and Laws from the University of Melbourne. Before entering parliament, he was a principal solicitor for Slater and Gordon and as a senior associate at Holding Redlich lawyers and consultants.^[3] He acted as a solicitor for refugees stranded aboard the MV Tampa.^[5] Giles also worked as a senior advisor for the Bracks and Brumby Governments in Victoria.^[3] He was secretary of the Socialist Left in Victoria.^[6]

Giles was elected to parliament at the 2013 federal election, replacing the retiring Harry Jenkins in the Division of Scullin. He immediately took up a position as Labor's cities taskforce and has engaged in an Australia-wide cities listening tour.^[7] He is co-chair of Parliamentary friends of Amnesty International, and the deputy chair of the Parliament's Northern Europe group of its IPU members.^[8]

In 2016 he was elevated to the position of the Shadow Assistant Minister for Schools. In 2019 he became a member of Labor's Shadow Ministry. His portfolios included: Multicultural Affairs, Cities and Urban Infrastructure. He also served as the Shadow Minister Assisting for Immigration and Citizenship.^[9] In this role Giles led Labor's campaign to stop the Morrison Government's attempts to privatise Australia's visa system.^[10] He was also responsible for Labor's ‘reboot’ with multicultural communities, including the establishment of a Multicultural Caucus Committee to inform future policy development.^[11]

Giles has two children and lives with his wife, Dr Jillian Constable.^[1] In the 1990s, he played guitar in an indie rock band called Ether, along with James Cecil, who went on to play in bands such as Architecture in Helsinki.