Dennis was born in 1949 and grew up in Chicago in a blue-collar family. At 17 he took a job as a trading-floor runner at the Chicago Mercantile Exchange. After high school he earned a philosophy degree, was accepted to graduate school on scholarship, and dropped it to trade. With $1,600 borrowed from his family — $1,400 of which bought him a seat on the MidAmerica Commodity Exchange, leaving him with $400 in actual trading capital — he started running positions in soybeans and grains. By the mid-1970s he had reportedly made $350 million from that initial $1,600. In 1986 alone, by his own accounting, he booked $80 million (roughly $147 million in 2007 dollars).

The Turtle experiment is what made him famous outside the trading world. In December 1983 and December 1984, Dennis ran a public newspaper ad recruiting people with no trading experience and trained 23 of them — two cohorts, the "Turtles" — in two weeks on a simple trend-following system: buy when prices break above a recent range, sell when they break below it, sized by volatility. The experiment was a bet between Dennis and his friend William Eckhardt over whether successful trading could be taught. By the time it ended five years later, the Turtles reportedly had earned an aggregate profit of $175 million, and several of them (Jerry Parker, Curtis Faith, Liz Cheval) became major CTAs in their own right.

Dennis lost meaningful capital running OPM in the 1987 crash and quietly retired from active trading for several years. He closed C&D Commodities in 1996 and has been largely absent from public trading life since, choosing philanthropy, Democratic Party donations and Libertarian-leaning policy work instead. For trading-tournaments.com, Dennis is a Hall-of-Fame profile — the historical proof that systematic trading is teachable, and the source-of-source for every modern trend-following fund.