After Howard's defeat at the 2007 election, Sinodinos spent time working for both Goldman Sachs and the National Australia Bank, before being appointed to fill a vacant seat in the Senate in 2011.

After the Liberal victory at the 2013 election, newly-elected Prime Minister Tony Abbott appointed Sinodinos Assistant Treasurer.

Sinodinos later stepped aside from his ministerial duties in 2014 during an investigation by the New South Wales Independent Commission Against Corruption (ICAC).

[6] His father was a member of the left-aligned Seaman's Union that, during Sinodinos' early years, was campaigning against the United States intervention in Vietnam.

His mother recalled stories of the Greek Civil War of the 1940s where Communist insurgents would knock on the door of their family home at night.

[6] He was the Chief of Staff to then Prime Minister John Howard from 1997 to 2006, after serving with him previously from 1987 to 1989 when he was Opposition Leader and rejoining him in 1995.

[13][14] Sinodinos was appointed to fill the vacant New South Wales Senate seat opened by the resignation of Helen Coonan on 13 October 2011.

[20] On 19 March 2014, Sinodinos stood aside from his role as Assistant Treasurer prior to giving evidence as a witness before New South Wales's Independent Commission Against Corruption (ICAC).

During the enquiry Sinodinos advised he was unaware of a $74,000 donation made to the Liberal Party by Australian Water Holdings, despite being Deputy chairman on a $200,000 salary.

[32] Sinodinos was appointed an Officer of the Order of Australia in 2008 for service to politics through the executive function of government, to the development of economic policy and reform, and to the Greek community.