Farrell was elected as the ALP's deputy Senate leader in 2016 and is a senior figure in the Labor Right faction.

[2] His father was a member of the Democratic Labor Party (DLP) and stood for federal parliament unsuccessfully on six occasions, five times for the seat of Boothby and once for the Senate.

[2][3] During his childhood, Farrell lived for periods in Crafers, Forestville, Panorama, and Daw Park.

[1] Farrell joined the Shop, Distributive and Allied Employees Association (SDA) as an industrial officer in 1976.

He was a delegate to the Australian Labor Party National Conference from 1984 and was elected state president of the ALP in 1988.

In 2011 Farrell was listed as number six on the top ten political fixers as identified by The Power Index website.

He stated that he would demand that Labor's national executive overturn the decision and promote Senator Wong to the number one spot.

On 30 October 2012, Farrell stepped aside to give Wong the number one spot on Labor's senate ticket for the 2013 election.

This is a case whereby modern Labor can't be seen to be concentrating on our internals at the expense of what is going on out there in the electorate and what is expected."

Wong later expressed favourable sentiments about Farrell after his decision to stand aside:[13]"Throughout his career, Don has always put the Labor Party first and he has demonstrated that principle again today.

[17] In 2016 Farrell was endorsed as a Labor candidate for the Senate in South Australia in the 2016 federal election,[18] and was subsequently re-elected.

[27] His goal of securing a free trade agreement with the European Union had eluded him, with the Minister leaving a meeting with his EU counterpart in 2023.