On his fourth attempt to enter federal parliament in 2013, Day was elected to the Senate from a 3.8 per cent primary vote for Family First,[3] to a term beginning on 1 July 2014.

[5] Elected to the 12th and final South Australian Senate spot, Day's term was due to expire on 30 June 2019.

[7] However, he stated that a potential new investor had expressed interest in the business, that there was too much important work for the Family First Senate seat to be vacant for even one day and that therefore he would not resign before the year's end.

[9] On 5 April 2017 the High Court held that Day's re-election to the Senate in July 2016 was invalid, since he'd had an "indirect pecuniary interest" in an agreement with the Commonwealth since at least February 2016.

[14] Day is the sole director of Ashford Homes, which is also facing action over unpaid money to creditors.

In September 2016, The Australian newspaper raised concerns of how Day contributed $380,000 to Family First in 2012–13 while his businesses appeared to be facing financial difficulties.

[19] In his community service role, Day has planted several thousand trees for farmers and land owners across South Australia.

He undertook a roadside planting, irrigation and re-vegetation program along North East Road including a local school.[when?

He had joined Family First immediately prior to the by-election, after resigning his 20-year membership of the Liberal Party, accusing the party of a "manipulated" process which saw former Howard government advisor chiefly for WorkChoices, Jamie Briggs, gain Liberal preselection at the expense of others including Day.

Some commentators claimed Day had a "strong chance of taking one of the last two South Australian Senate seats", citing "effective preferences from nine smaller parties".

[25] Other commentators rated Day a "slim" chance, citing campaign and financial troubles with the Family First Party.

[28][29] However further progression of the count put Fawcett back in the lead by several thousand votes and went on to win the sixth and final South Australian Senate seat.

However upon Edwards and then One Nation candidate Steven Burgess being eliminated at count 445 and 455 respectively, leaving only McEwen and Day remaining, Day had collected enough preferences to overtake and narrowly defeat McEwen − by a couple of thousand preference votes.

[6] On 17 October 2016, administrators McGrathNicol were called in to liquidate the Home Australia Group, a building company founded and managed by Day.

This gave Family First time to identify a replacement senator, and meant that Day could vote on contentious legislation such as the Australian Building and Construction Commission bill that had been one of the triggers for the double dissolution in 2016.

In a statement, Day said "while a number of offers for various parts of the Home Australia business have been received, the major investor who has been examining the group's portfolio of assets over the past fortnight, has decided not to proceed.

The government believed Day had potentially violated section 44(v) of the Constitution, which provides that a person who "has any direct or indirect pecuniary interest in any agreement with the Public Service of the Commonwealth" is "incapable of being chosen or of serving" in either house of Parliament.

At least three judges were prepared to say that the violation dated to 1 December 2015, when the Commonwealth initially entered into a lease agreement with the building's owner.

This was despite the fact that Commonwealth public servants advised Finance Minister Mathias Cormann that it was open to him and that it would not be a conflict to make rental payments under the lease from 1 March forward (Hansard 7 November 2016).

Though it had no formal affiliation with any particular religious organisation, Family First was strongly linked to the Pentecostal church in South Australia, and nationally from smaller Christian denominations.

[68] Both Day and Leyonhjelm have long been members of the free market think tank Institute of Public Affairs (IPA).

[69] Day advocates sustainable, balanced land and water use which takes into account Australia's scarce resources.

He was the first president (2001–05) of Independent Contractors Australia, which was formed in 1999 "with the objective of protecting the rights of independent contractors in Australia to be treated fairly, justly and equitably and to be allowed to work free from intimidation or harassment from bureaucrats, the Australian Taxation Office, political parties, unions and others".

Nicholls, he blamed the award system for high unemployment and the social ills of drugs, crime, violence, poor health, teenage pregnancy and suicide.

There would be no industrial relations commission and workers could settle disputes through either voluntary mediators or magistrates courts.

Yet last week The Courier, a local paper in Mayo, featured a small interview piece with Day.

"Even on Work Choices – the controversial industrial relations reform that was the biggest single factor in the Coalition's federal election loss – Mr Day said he shared the same views as his new party, which opposed the unpopular policy."

"It's true to say his position was to oppose it but only because he thought Work Choices was too bound up with regulation and red tape," said one former colleague.

"[24]In May 2017, federal Employment Minister Michaelia Cash referred to the Fair Work Ombudsman allegations that Day had claimed that staff at Homes Australia were independent contractors on commission-only pay when they were in fact employees and therefore had substantial additional legal entitlements.

They formed proprietary limited companies to take advantage of all the benefits of being incorporated for tax purposes.