“This is a masterpiece of midcentury architecture, and we’re going to be reactivating it with a new curatorial spirit and reweaving it in new ways into the cultural fabric of New York City,” he said, standing on the museum’s garden level, at the base of what the critic Ada Louise Huxtable once described as the building’s “disconcertingly top-heavy, inverted pyramidal mass,” a shape that grew on one only slowly, she added, “like a taste for olives or warm beer.”

But the building — which the Met has leased from the Whitney for eight years and which opens to the public on March 18 — has long since taken its place within the city’s architectural DNA. And in deciding how to reinvent it for a new museum and a new age, John H. Beyer, a founding partner of Beyer Blinder Belle, the architecture firm that restored the building, said that most of the decisions involved simple, originalist steps backward, to Breuer’s intentions. The bookstore was removed from the northern end of the lobby to open up the floor so visitors immediately sense the broad span between the museum’s load-bearing walls. A partial wall concealing the guts of the old automated coat rack was also removed, Mr. Beyer said, “because that coat rack was as much a part of Breuer’s design as the architecture — he loved it; it was new technology.”

The building’s few basic materials — concrete, granite, bluestone pavers, bronze for handles and railings and elevator doors — were taken almost back to their elemental states, with only a low-sheen wax applied to the floors. The wood railings were stripped of varnish and finished with Danish oil, as Breuer specified, so that they exude a sense of texture that the architect described as being “close to earth.” The rough, bush-hammered concrete walls, which reveal the black obsidian stones used in their making, were only cleaned and holes in them were plugged, sometimes with obsidian, to mimic the texture around them.

“He loved the hard dignity of aging materials,” Mr. Beyer said. “One of the most important parts of restoration is in deciding what not to do. And we didn’t do a lot. But what we did, I think, gives you a clear understanding of the space as he created it.”