While at school, Coates started work in the cashiers' department within the family bookmaking firm, Provincial Racing, where she would mark up bets. After leaving university she trained as an accountant within the family Business, before being given the opportunity to take over the small chain of shops. Having turned around the shops' fortunes, Coates obtained a loan from Barclays to acquire a neighbouring chain. In 1995, she became managing Director of the betting shop estate.
In January 2000, Coates purchased the domain name Bet365.com. While working in a temporary building on a car park near one of her shops, she spent a year creating a new online betting site. Bet365.com was launched in 2001. The Business borrowed £15 million from RBS against the family's betting shop estate. In 2005 these shops were sold to Coral for £40 million which also paid off the loan to RBS.
In 2013, Coates was named as one of the 100 most powerful women in the UK by Woman's Hour on BBC Radio 4.
They have five children, including four that in March 2014 were reported as being "recently adopted from the same family".
Bet365 has grown into one of the world's largest online gambling companies, with $2 billion in revenues and facilitating $45 billion in bets in the year through March 2016. The company also owns a majority stake in the Stoke City Football Club. In 2015, Bet365 moved its headquarters to Gibraltar because of its favourable regulations. Coates still runs the company alongside her brother, and co-CEO, John Coates.
In 2017, Coates was criticised for paying herself £217m, with Mike Dixon, CEO of Addaction, saying "It cannot be right that the CEO of a betting company is paid 22 times more than the whole industry ‘donates’ to treatment."
Denise Coates is the majority shareholder with 50.01% of Bet365. Her personal fortune is estimated at $5.1 billion, as of February 2018.