Beamon arrived as a strong favourite for long jump gold. He had won 22 of 23 meets that season, one with a world-leading distance of 8.39 metres (wind-assisted).
However, he nearly failed to make the final after overstepping on his first two of three qualifying jumps - he jumped from well behind on the third attempt, and did just enough to get through.
So nervous was Beamon that he spent the night before the final drinking tequila, but there was no sign of a hangfover when he set off on the runway for his first jump.
The problems in qualifying melted away, and Beamon hit the board perfectly, soaring through the thin Mexican air and landing so far into the pit that his distance could not be measured using the modern optical device.
So the judges called for a tape measure, and some 20 minutes after Beamon's historic jump, the scoreboard flashed up: 8.90m.
Beamon had broken the world record by a preposterous 55cm - nearly two feet. Competition over. Neither Beamon nor anyone else got close over their next five jumps.
The American wore black socks on the podium as a tribute to Tommie Smith and John Carlos, who were sent home following their black power protest.
His record stood 23 years, falling to Mike Powell at the 1991 World Championships in Tokyo.