Leahy was speaking at the Marketing Society's annual conference, which kicked off today, when he also said that marketing as a profession has lost much of its confidence and was more averse to the risk-taking needed to innovate.
The former Tesco chief executive was onstage with entrepreneur Michael Welch, whose Blackcircles.com online tyre-fitting business Leahy has invested in.
Asked about Tesco Clubcard, Leahy said: "It’s probably the first example of big data, which is the buzzword of today. I pinched the idea of Clubcard from the Co-op, where I started my career."
At Tesco, he took the idea to the bosses who thought it was a bad idea. He said: "We got more data, bided our time and went back again."
Discussing the concept of innovation and entrepreneurialism, Leahy stressed the need for people to take risks, and cited marketing as a discipline that was lacking that quality.
He said: "I think marketing has lost its confidence as a profession. Marketing people have to be braver and take more risks. You might get fired, but you've got to take risks."
He added that one of the best places to innovate from within an organisation was the marketing department.
Leahy said: "When you innovate it's not always going to work. Innovation and failure are two sides of the same coin."