 Before entering the Senate, Farrell was the South Australian Secretary and the National President of the Shop, Distributive and Allied Employees Association (SDA) union and a leader of Labor Unity in South Australia.Farrell first ran for parliament at the 1988 Adelaide by-election but was unsuccessful.In June 2007, Farrell won preselection for the first position on Labor's Senate ticket in the 2007 election and he was subsequently the first elected senator for South Australia at the general election.