His role in the stunning victory for that campaign made him a hero to many Brexiteers but a hate figure for some Remainers.
The 48-year-old has been credited with creating the "take back control" slogan of the Vote Leave campaign and criticised over the which was advertised on the side of the Brexit bus which travelled the country.
Many will know him as the character played by Benedict Cumberbatch in the HBO/Channel 4 comedy-drama film Brexit.
The appointment of the abrasive former campaign director was controversial especially given he had been found to be in contempt of Parliament earlier in the year for refusing to give evidence to MPs investigating misinformation.
He would later say the pledge was "necessary to win".
The campaign group was also fined £61,000 for breaking the rules in the build-up to the vote.
Mr Cummings is variously seen as a genius, a maverick, or as a source or ire.
He was once labelled a "career psychopath" by former prime minister David Cameron, according to widely-reported remarks.
But Mr Cummings is not shy of firing off an insult himself.
In 2017, he described David Davis, then the Brexit secretary, as "thick as mince, lazy as a toad and vain as Narcissus".
The December 2019 election victory gave Mr Johnson the political capital he needed to take bold decisions – and Mr Cummings soon set to work on his goal of reshaping Whitehall, issuing a recruitment call for data scientists, economists and “weirdos and misfits with odd skills” to shake up the Civil Service.
Downing Street insisted there was nothing untoward about his attendance, but opposition MPs and some scientists suggested it risked political interference in science-based advice from Sage.
Mr Cummings also drew criticism when he returned to work after his own brush with Covid-19 and was pictured allegedly failing to follow the two-metre social distancing rules as he walked along Downing Street flanked by fellow aide Cleo Watson on April 14.
Mr Cummings' importance to Mr Johnson's government was shown over the weekend in how heavily he was defended by many Conservative MPs following the allegations that he broke lockdown rules.