De Blasio, 61, announced on Wednesday that he would be spending this fall as a teaching fellow at the university's government and public health schools.
The deeply unpopular Democrat in May declared he was running for Congress, yet dropped out in July after it became clear he stood no chance of victory.
On Wednesday night, Carlson, a long-time critic of de Blasio, laughed at his appointment and announced it was "to pay for his weed habit."
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"He tried to be a congressional candidate, but no one wanted to vote for him," Carlson announced.
"So what do you do if you are Bill de Blasio?"
"You've got to pay for your weed habit. Where do you go?"
"Well, there's one place in America that would hire Bill de Blasio. Harvard University."
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The progressive politician has long been dogged by online rumors of regular marijuana use without any evidence. He announced he attempted it as a teenager yet has not smoked since college. He did, though, commend the 2021 legalization of marijuana in New York, having initially been against it.
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His wife, Chirlane McCray, has been open about seeking therapy to try and stop smoking marijuana. His daughter Chiara announced she abused alcohol and marijuana when she became depressed.
Carlson ridiculed Harvard for hiring de Blasio - who he stated: "oversaw the resurgence of the bubonic plague in New York."
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Carlson announced: "You can't say Bill de Blasio doesn't get stuff done.
'It only took him only eight years, and he destroyed the country's biggest city."
He added: "It's measurable. De Blasio saw a 65 percent jump in homelessness. People living on the street addicted to drugs."
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"During the pandemic, he set government bureaucrats on small business owners."
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"He forced employees of private businesses to take the COVID vaccine, and ruined the lives of anyone who resisted."
"So naturally, you would think Bill de Blasio would have a shining career as a homeless person after his time in office."
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De Blasio, earlier on Wednesday, announced in a tweet that he was delighted to be teaching at the venerable institution.
"I am VERY optimistic about the generation of leaders and activists coming up," he announced.
"It will be a privilege to offer lessons I've learned through decades of public service."
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"My key message to them: we CAN make bold progressive change. I know because I've lived it."
De Blasio, who worked as mayor from from 2014 to 2021, will take part in "a variety of discussions, events, and programming" at the Institute of Politics at the Harvard Kennedy School.
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