After attending Rainham Mark Grammar School, he studied zoology and comparative physiology at the University of Birmingham, graduating with a Bachelor of Science (BSc) degree.

[11] After hearing Alan Sked, a professor at the London School of Economics, speak on a BBC politics programme,[12] he was persuaded to stand at the 1992 general election as an independent in support of the Anti-Federalist League, receiving 248 votes in Gillingham.

After extensive wrangling, Jeffrey Titford was elected as the new party leader; Mackinlay stepped down as deputy, but remained on its NEC.

[16] Mackinlay remained active in UKIP, standing in Totnes at the 2001 general election, at which he received 6.1% of the vote,[18] then back again in Gillingham in 2005, where he polled 2.6%.

In the November 2012 county-wide poll, he lost to the former Kent Police Authority chair Ann Barnes, by a 114,137–60,248 margin, on a turnout of just under 16%.

In October 2017, Mackinlay said "unemployed young people from Glasgow should get on their bikes and work with gorgeous EU women on farms in the south of England after Brexit."

[26] In June 2018 it was reported that Mackinlay had been found to have twice breached parliament's rules due to a potential financial interest, according to a decision by its standards watchdog.

Mackinlay responded that he no longer had plans for running flights from Manston Airport and that there "was no suggestion he benefited financially from raising the matter in the Commons.

[28][29] Following an interim report on the connections between colonialism and properties now in the care of the National Trust, including links with historic slavery, Mackinlay was among the signatories of a letter in November 2020 to The Telegraph from the "Common Sense Group" of Conservative Parliamentarians.

They have argued for fracking in the United Kingdom to be resumed and cast doubt on the viability and desirability of a planned phase-out of fossil fuel vehicles.

[citation needed] In a court case on 1 June 2016, brought by Mackinlay and his election agent Nathan Gray,[36] District Judge Barron granted more time for investigation saying "In this case, the allegations are far-reaching and the consequences of a conviction would be of a local and national significance with the potential for election results being declared void.

[38] The day after, Channel 4 News published leaked emails, alleging that Theresa May's Political Secretary Stephen Parkinson, and Chris Brannigan, Director of Government Relations at the Cabinet Office, also took a key role in Mackinlay's campaign.

[43][44] Later in August 2017, a trial date of 14 May 2018 was set for Mackinlay on charges relating to his 2015 general election expenses, alongside Little and Gray.