He will likely go down in corporate history as the most accomplished British manager of his generation, the business he built as certainly the country's most successful retailer and arguably its best-run company, full stop. Group operating profit has risen 340% since he took over and Tesco is now the world's third-largest superstore outfit.
It will be the end of an era for Britain's Most Admired Companies, too. Under his leadership, Tesco has won the coveted overall award an unmatched six times, in 1996, 1998, 1999, 2001, 2005 and 2006. It has wiped the floor with its nearest rivals for the most honours garnered - Shell, Cadbury and M&S. These three runners-up, no BMAC slouches themselves, have managed only a pair of wins apiece.
But Leahy has an even more impressive personal track record - including this year's win, he has been named Most Admired Leader for no fewer than eight of the past 10 years.
And yet Liverpool-born Leahy is unassuming, low key - yes, even a tiny bit dull. He lives quietly with his GP wife Alison and their three kids and eschews the trappings of success. He doesn't seem to need mega-yachts or flash cars to massage his ego. But he does love to talk shop, especially with customers and staff.
The product of a UMIST management course, Leahy famously failed to get hired by Tesco the first time he applied, but joined the firm at second bite, becoming a marketing exec in 1979. At the time, Tesco was playing catch-up, a distant third to M&S and Sainsbury's. Leahy's big idea was that it should stop copying its rivals and start to make its own decisions based on superior customer knowledge. This led to the Club Card loyalty scheme, which enabled Tesco to boost sales dramatically by analysing the mountain of data it provided.
His run at the top has not been without controversy - Tesco is the subject of gripes that it squeezes suppliers so hard on price that only the hardiest can survive its embrace. It has also been accused of damaging local communities with out-of-town developments, and of doing its best to circumvent planning restrictions when siting new stores. None of which seems to have deterred shoppers from flocking there in their millions.
But it's overseas expansion that will arguably be Leahy's greatest legacy - the firm now has over 5,000 stores, half of them outside the UK. It operates in 14 foreign markets from China, Japan and South Korea to India, Poland and the Czech Republic. However, Fresh'n'Easy in the US is making very slow progress. By April 2011 there will be about only 50 stores stateside.
Nevertheless, it will be an enormous task for successor Phil Clarke to fill Leahy's shoes - anxieties about which may be reflected in Tesco's rare excursion below the BMAC top five this year.