The venerable Curtis Center at Sixth and Walnut Streets, a landmark of Philadelphia's publishing past, was purchased Monday for $125 million in cash by Keystone Property Group and Mack-Cali Realty Corp.

Keystone, of Bala Cynwyd, and Mack-Cali, of Edison, N.J., said the building, which covers an entire block and overlooks Washington Square, will become "mixed-use," as 90,000 square feet of unoccupied office space is converted to luxury rental apartments.

More residential space could be developed in the Curtis Center as more office space becomes vacant, Mack-Cali said.

The 12-story, 885,000-square-foot building has been owned since 2006 by Apollo Global Management L.L.C. of New York, which paid $94 million for it, property records show.

The Curtis Center is home to a number of commercial tenants and ground-floor retailers. Its anchor tenant is the U.S. General Services Administration.

The plan to turn a portion of the building's office space into luxury rentals is the latest example of the expansion of Center City's high-end apartment market as the for-sale housing side struggles to return to normal.

On the other hand, the Curtis Center is only one block west of developer Tom Scannapieco's planned 26-story, 40-unit condo tower at Fifth and Walnut Streets.

As of mid-May, that high-rise, to be built at a cost of $150 million for a spring 2017 opening, was already $40 million presold, Scannapieco said. Construction is slated to begin next spring.

The Curtis Center was built between 1909 and 1920 by publisher Cyrus H.K. Curtis as headquarters for his publishing empire, which at one point included the Saturday Evening Post, Ladies' Home Journal, and, for a few years, The Inquirer.

The structure underwent $80 million in renovations in 1980, which included the refurbishing of office space, common areas, and building systems, among other improvements.

The buyers are planning corner restaurants that front Sixth and Walnut, as well as improvements to the elevators; the heating, ventilation, and air-conditioning systems, and the roof and facade. The 350-space underground parking garage will also be renovated and the Tiffany mural of Dream Garden by Maxfield Parrish will be kept.

In a statement, Mack-Cali president and CEO Mitchell E. Hersh said the acquisition strengthened his company's presence in the region's multifamily housing market.

Keystone president Bill Glazer said the Curtis Center purchase is aligned with his company's goal of expanding its "regional footprint."