In the early 21st century, the building underwent an extensive conversion into office and retail space, and is most well known as the former headquarters of Twitter, Inc. and its successor X Corp. until 2024.

[3] Additionally, the top floor served as the headquarters for local radio station KSAN, with a large broadcast antenna fixed atop the building.

[3] Several days later, on August 3, the mart officially opened, with the keynote speaker for the event being Executive Vice President Roscoe R. Rau of the National Retail Furniture Association.

[3] During World War II, an outdoor patio was reconfigured into a warehouse, with Douglas fir wood being used in place of steel and concrete due to wartime materials rationing.

[7][8] In 1985, given the poor state of the Mid-Market, columnist Herb Caen of the San Francisco Chronicle called the area "Le Grand Pissoir",[4] while a 2022 article in The San Francisco Standard called it "a forlorn cityscape of half-empty buildings, struggling storefronts and troubled people living on the streets".

[3] During this time, a semi-annual trade show that was held at the mart relocated to the World Market Center Las Vegas, while many high-end retailers moved to other boutique venues in San Francisco.

[3] In January 2008, the company gave notice to the remaining vendors that they would have to vacate the building by the end of the year, after which it would undergo a conversion that was estimated to take about 18 months.

[9] This conversion process coincided in part with the Great Recession,[3] which left about half of Mid-Market's offices and 30 percent of the neighborhood's retail centers vacant.

[6] According to Twitter executive Colin Crowell, the company, which was planning to double their staff, did not believe that remaining in San Francisco was optimal given the city's payroll tax.

[1] In 2015, Shorenstein began to seek offers for the property,[16] and in August of that year, they sold a 98 percent stake in the building to the asset management division of JPMorgan Chase, with a valuation of $920 million.

[19][20] In 2017, a new skyway was added to connect Market Square to Mart 2, as the previous bridge had been removed in 2011 due to not being compliant with the Americans with Disabilities Act of 1990.

[21][22] In April of that year, prior to the acquisition, Musk had tweeted about converting Twitter's headquarters into a homeless shelter, which generated some support from fellow business magnate Jeff Bezos.

[23] In December of that year, following the acquisition, the New York Post reported that Musk could relocate Twitter's headquarters from San Francisco following an incident wherein the city launched a probe into possible zoning violations performed by the company.

[22][27] That same month, a lawsuit was filed in a California superior court alleging that the company had failed to pay rent for both December 2022 and January 2023 on the Market Square property.

[33] Prior to this, the San Francisco Police Department had stopped workers from removing a Twitter sign off of the building because they had not properly secured the worksite before beginning the dismantling.