Austin was elected as a councillor in the Metropolitan Borough of Dudley in 1991, and served until 1995. He then moved to become press officer for the West Midlands Labour Party until 1998, when he spent a year as Deputy Director of Communications for the Scottish Labour Party.
In 1999 he was appointed a political advisor to the Chancellor of the Exchequer (now Prime Minister), Gordon Brown. He held the position until his election in 2005, and is well known as one Gordon Brown's closest lieutenants.
Austin was reprimanded by the Speaker of the House of Commons for heckling during Prime Minister's Questions on 18 October 2006, and he was subsequently described by David Cameron as one of Gordon Brown's "boot boys". The following week he was rebuked again by the Speaker for comments made towards the Conservative benches.
Following Gordon Brown becoming prime minister on 27 June 2007, Austin was tipped for a post in Brown's inner circle. The following day he was appointed a Parliamentary Private Secretary to Gordon Brown with a special provision to attend cabinet.