The Mexican Museum — its attendance in decline and its rent in arrears — will close its longtime galleries and offices in the Fort Mason Center for Arts & Culture this fall.

The private nonprofit has not paid its rent since October and will forfeit its 5,000-square-foot home in Landmark Building D, where it has been since 1982, in exchange for forgiven rent of $140,000. It will vacate in November, said Andy Kluger, chairman of the board of trustees.

The museum will not reopen until 2020 at the earliest, as it cuts overhead and conserves cash. It is racing to raise $30 million required by the city to build a new Mexican Museum in the 706 Mission St. condo tower under construction near the San Francisco Museum of Modern Art.

The four floors of condo space, which occupy 60,000 square feet including ground-floor restaurant and retail space, belong to the city and county of San Francisco, which has signed a 99-year lease with the Mexican Museum at a rate of $1 per year. But that is for an empty shell. The museum must pay for all tenant improvements and show the city that it has access to $30 million to gain occupancy.

The museum is separately trying to raise an additional $30 million as an endowment to cover its operating costs. In an interview with The Chronicle, Kluger was able to produce a typed document indicating the capital campaign has pledges of $9 million, plus $5 million from the tower developer, toward the $60 million total.

Kluger also said the city, through the Office of Community Investment and Infrastructure, has allocated $7.5 million toward interior tenant improvements with an additional $7 million pledged, or nearly half the $30 million needed.

But Max Barnes, spokesman for the agency, said total construction grants to the Mexican Museum could not exceed $10.5 million. In interviews, city arts and real estate officials said the museum has received $12 million in arts grants to keep it afloat over the years but has promised nothing beyond the $1-a-year lease.

Nothing can be counted for sure because the Mexican Museum has not submitted audited financial statements to the city for 2016 or 2017. The city sent the museum letters in April and May requesting the audited statements, and Kluger said his attorney is working on it. The audit will document pledges toward the capital campaign.

“We’re comfortable. I’m not panicked,” Kluger said. “It looks like we will be there pretty soon.”

The developer, 706 Mission Co., an entity owned by Westbook Partners, plans to complete the shell in 2019. Kluger already has a design for the museum including galleries and pledges that the museum will open in 2020.

Both deadlines sound optimistic given that the 45-story tower has not yet risen above ground level, and the museum has not yet demonstrated that it can privately raise anywhere near the $30 million it needs to build the shell into the international museum that is planned.

The project is to meld the historic 10-story Aronson Building into a new condo tower. In a deal cut in the early 1990s with the city redevelopment agency, the ground floor plus three above it, in the combined project, was deeded to the city as space to install a cultural institution.

The Mexican Museum was promised the space from the start, and the 99-year-lease has been signed, according to Tom DeCaigny, director of cultural affairs for the San Francisco Arts Commission, which is overseeing the city’s end of the deal.

“We are committed to working with the Mexican Museum to see it through,” DeCaigny said, emphasizing the $12 million in city grants. Now the onus is on the museum.

“They need to show the $30 million in tenant improvements to us to get in the door,” DeCaigny said. He would also like to see a museum director hired. It has been without one for many years, with the museum being run by its chief operating officer.

Kluger said a search committee in Mexico City has identified 12 candidates for the position of director, and the board will soon be interviewing candidates.

Kluger, a San Rafael investor, has been the board chairman for six years, during which the Mexican Museum has attempted to elevate itself from a small community museum in a makeshift gallery space that has always been temporary to a museum that holds its own in the city’s cultural center.

In 2012, it was accepted as a Smithsonian affiliate, which means it is eligible to lend and receive artifacts and exhibitions administered by the Smithsonian Institution. In 2014, it completed the Museum Assessment Program administered by the American Alliance of Museums.

The museum’s collection has nearly 17,000 objects, many of which were donated over the years with their authenticity unverified. In advance of its move, the museum began a two-year authentication process last year. It ran into trouble right away when it was revealed that artifacts from the pre-Columbian, or pre-Hispanic, era could not be authenticated.

In the end, an estimated 300 pieces were authenticated and another 500 could not be. The pre-Columbian process is now complete, and the museum is starting to authenticate its folk art, 5,000 pieces.

To improve the quality of the collection and reduce its storage expense, the museum is de-accessioning anything that cannot be authenticated along with objects considered “decorative.” In May and June, Clars Auction Gallery in Oakland sold a large collection of works from the Mexican Museum.

One former board member, who declined to be named, sent The Chronicle documents to indicate that various art objects he had donated to the museum, with a certified appraisal value of $53,000, had recently been sold through Clars for a total of $7,777, or 14 percent of their market value.

“The things that are selling at Clars are whatever the market will bear,” said Theron Kabrich, co-owner of the San Francisco Art Exchange and a Mexican Museum board member. “Many of those items are duplicates or not appropriate to the collection.”

But there are also new donations coming in, including a huge trove from collectors John and Kati Casida that has been appraised at $4 million, Kluger said. “It will be one of the principal showpieces at the new museum,” he said.

Kluger boasts about this in his pitch to investors. He’s made 19 trips to Mexico and carries a glossy 32-page brochure that includes the architectural sketch of all four floors, with naming opportunities, for $3 million each.

“We have people who said, ‘We’ll give,’” Kluger said. “Now we want to see action.”

At this point, the only art that seems certain at 706 Mission is a two-story public art facade that will wrap around the space reserved for the Mexican Museum. It was commissioned to Jan Hendrix, a Dutch artist who lives and works in Mexico. The facade’s $2 million cost will be paid for by the developer as its percent for art requirement.

The terms of the deal at 706 Mission are only that the city-owned condominium be used for cultural purposes. It was never specified that it had to be the Mexican Museum. Others would no doubt be interested in 99 years at $1 per, but DeCaigny said no other cultural institutions have been considered.

“The museum’s collection, particularly Chicano art, has deep ties to the Bay Area,” he said. “We want to make sure we have a place where the collection is accessible.”

In 2015, the last year it filed an audit, the Mexican Museum claimed total net assets of $2.1 million. In June, its attendance was 316, down from 376 in June 2017. When it closes in November, museum staff will move to office space it has arranged with a nonprofit near its new home at Third and Mission streets.

“This turn of events will help us aggregate more funds quicker toward our goal to open on time in 2020,” Kluger said.

Also planning to leave Fort Mason in a few years is the Museo Italo Americano, now in its 40th year. The museum has been gifted a commercial warehouse on Battery Street near the waterfront, where it will relocate pending a capital campaign and city entitlements.

Sam Whiting is a San Francisco Chronicle staff writer. Email: swhiting@sfchronicle.com Twitter: @SamWhitingSF Instagram: @sfchronicle_art