The moment came as Sanders swore in Mamdani, who described himself in his inaugural address as a "Democratic socialist," telling supporters, "I was elected as a democratic socialist and I will govern as a democratic socialist."
Sanders' appearance marked a visual departure from the image that made him a global pop-culture figure four years ago, when photos of the Vermont senator bundled in a winter coat and handmade mittens spread rapidly across social media during Biden's inauguration.
Before administering the oath, Sanders delivered a lengthy address praising Mamdani's campaign and agenda, framing the election as a rejection of political establishments.
He told the crowd that volunteers had "took on the Democratic establishment, the Republican establishment, the president of the United States and some enormously wealthy oligarchs."
Sanders also defended policies Mamdani's opponents had labeled "radical" or "communistic," arguing that proposals such as affordable housing, free childcare, free bus transportation and taxing the wealthy were "not radical" but necessary in "the richest country in the history of the world."
Sanders formally administered the oath of office shortly after 2:40 p.m., prompting chants of "Zohran" from the crowd gathered at City Hall.
Mamdani pledged to pursue policies including freezing rents for stabilized apartments, delivering universal childcare, making buses free and increasing taxes on wealthy New Yorkers, saying City Hall would "govern expansively and audaciously."
Sanders' attire drew widespread attention in 2021, when images of him sitting alone wearing brown chevron mittens with his arms and legs crossed became one of the most widely shared visuals of Biden's inauguration, spawning memes, merchandise and fundraising efforts.
During Thursday's inauguration ceremony, a similarly cross-legged Sanders showed up sitting patiently in the audience during the performance of the socialist anthem "Bread and Roses" by Lucy Dacus.
On Tuesday, Sanders appeared in standard winter gloves, a blue beanie, and a puffy green coat instead.
The wool and fleece mittens were handmade by Vermont schoolteacher Jen Ellis, who later reflected on the viral moment in a 2022 interview with Vermont Edition, saying she donated about $2 million to Vermont charities through mitten-related fundraising following the internet sensation.