The Briton, who won 10,000m gold in London World Championships last weekend, was beaten by Ethiopia's Muktar Edris in the final of the 5,000m.
"I gave it my all, 110 percent", said Farah. "It's not an excuse, but it took a lot more out of me than I realized".
Quite simply, Farah will go down in history as Britain's best-ever athlete and one of the greatest the world has ever witnessed.
Edris won in 13 minutes, 33.79 seconds, .43 seconds ahead of Farah in second.
Mo Farah missed out on the 5,000m and 10,000m double at the World Athletics Championships, bringing down the curtain on his track career at major events with a silver medal. The better man won.
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Earth is set to once again pass through the dust of the Swift-Tuttle comet, sparking the annual Perseid meteor shower. The moon, which will be three-quarters full, will make it slightly more hard to see - as will viewing from the city.
America's Paul Kipkemoi Chelimo finished in 13 mins 33.20 secs to take bronze.
As a race it started fast, slowed to the proverbial crawl: Farah first hit the front with seven and a half laps to run, unusual for him, determined to dictate the pace than follow it. Five laps to go, Chelimo went again, then the Australian Patrick Tiernan, briefly opening a 10 metre gap, the tension rising all the time.
Down the home straight all looked lost for Farah as he appeared blocked in until Kejelcha drifted out from the inside lane trying to block Chelimo, which allowed the Briton the chance to battle through on the inside.
But when he starts his career as a marathon runner, Farah will be able to reflect on having created an awesome legacy with his four Olympic and six World Championship gold medals.