Jon Green fell for horse racing as a little boy tagging alongside his Uncle Marty to Monmouth Park on the Jersey Shore. Marty always wore three-piece suits, and after every race he would pull a winning ticket from one of his pockets for his nephew to cash in. Jon didn’t realize at the time that his uncle bought $2 tickets on every horse in every race.

Forty years later, Mr. Green has relied on business fundamentals rather than sleight of hand to make a colt named Sandman one of the favorites to win the 151st running of the Kentucky Derby on Saturday.

He took on partners, rebranded the stable he runs with his father, started a podcast and brought in a social media influencer with nearly 10 million followers to promote Sandman. In a shrinking sport that is losing its athletes and bettors, Mr. Green is among a new generation that is invigorating the game.

Now his business, DJ Stable, is a $30-million-a-year entity that breeds, buys and sells horses in Kentucky, New York and Europe. Mr. Green, who sold his financial planning practice, knows better than most the old racetrack maxim: How do you become a millionaire in horse racing? Start as a billionaire.

“You can make money in horse racing, maybe not every year, if you run it like a business,” said Mr. Green, who has turned a profit over the past seven years. “You’ve got to come up with a strategy and be disciplined in following it. It’s hard work. I went from managing $500 million and working 40 hours a week to a fraction of that and working 60 hours a week.”