Cementing its decision to shift its focus from the Upper East Side to the meatpacking district, the Whitney Museum of American Art on Thursday sold the six brownstones next to its signature Marcel Breuer building on Madison Avenue as well as two town houses around the corner on 74th Street.

The transaction makes it virtually impossible for the museum to expand in the neighborhood it has called home since 1966. “This is a major step toward the realization of a long-sought-after new facility that will better serve our artists and the community,” said Adam D. Weinberg, the Whitney’s director....