The 29-year-old defending champion carved out a half second advantage over Frenchman Alexis Pinturault through the 77 gates in his first run and nailed his second to claim victory by 0.65 seconds from compatriot Michael Matt.
It was Salzburg-born Hirscher's third world title in slalom having also bagged gold in 2013 and 2017, matching the record of Swedish great Ingemar Stenmark.
He also drew level with the men's record seven world championships golds held by fellow Austrian Toni Sailer.
After what had been a lean championships for the Austrians they at least finished in style, sweeping the slalom podium with Marco Schwarz taking the bronze, 0.76 seconds behind.
Olympic slalom silver medallist Pinturault's hopes of putting pressure on Hirscher disappeared when he made a mistake on his second run.
He made a remarkable recovery after sliding to the ground around a gate, but ended up fourth.
With such a margin in his favour Hirscher basically just needed to avoid a wipe-out in his second run to claim the title.
It was a conservative effort by his standards, only the 25th fastest of the second run, and Hirscher appeared to be tiring halfway down but he had enough in the tank to stay well ahead and take gold to go with his giant slalom silver.
Not a bad haul considering he was suffering with a heavy cold throughout the fortnight.
"I was much better than four days ago," who has nine world championships medals, said. "My team worked really hard to make this possible and get me in the start gate.
"I knew that Alexis was not in the lead so I tried to push really hard in the first part then there were a few gates that were bumpy so I tried to safe there, then attacked at the end."
Hirscher, hinting that this could be his last world championships, added: "After 2013 and 2017, it's unbelievable to have another gold medal."
Alpine powerhouse Austria ended with eight medals from the championships, more than any other nation, but it took another virtuoso performance from Hirscher to avoid the ignominy of going home without a gold.
It was a disappointing fortnight for hosts Sweden though as they managed just one silver medal.
(Reporting by Martyn Herman; Editing by Amlan Chakraborty and Christian Radnedge)
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