The 25-year-old -- a former builder which he credits for his body strength -- edged out America's defending champion Joe Kovacs, who posted a best of 21.66.
There was a heated moment, though, after Kovacs's final throw that appeared to be over 22m as the American punched the air with delight, only to see the red flag raised.
He demanded an explanation but there was to be no going back on the decision as his trailing foot clipped the board as he launched the shot.
Croatia's Stipe Zunic took the bronze with 21.46m whilst there was crushing disappointment for Olympic champion Ryan Crouser, who could finish only sixth with 21.20m.
Walsh pumped the air with his fist after his second throw and he had good reason to as he took the lead from Kovacs, who had hurled his shot out to 21.46.
Walsh consolidated his lead with his third effort recording a mark of 21.75m but things weren't going well for Crouser, who slipped to fifth by the third round.
The American's third effort a massive throw but to his consternation it was ruled a foul as Crouser wrung his fists in despair.
The pressure increased on Crouser as his fourth throw also resulted in a foul and he went into the sixth and final round in sixth spot.
However, there was to be no grandstand finale for the 24-year-old.
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