In 1916, Marjorie Merriweather Post, heir to the Postum cereal fortune, moved into her new home, a five-story, 54-room mansion at the southeast corner of Fifth Avenue and 92nd Street.

This was Millionaires’ Mile, a stretch of Fifth Avenue where the neighbors living in block after block of mansions were the richest, most powerful and socially prominent people in America. It could be a tough neighborhood. The social strictures were suffocating, while the reach for grandiosity knew no bounds.

Money had become more important than pedigree. And competition was fierce to build the biggest, most lavish home, borrowing English, French or Italian architectural styles while incorporating modern technology and dozens of servants.

Andrew Carnegie lived across 91st Street and Ms. Post’s neighbors included James Duke, F.W. Woolworth, the Astors, the Vanderbilts, Whitneys, Belmonts and Fishes.

Ms. Post was equal to the task of decorating a manse with 17 bathrooms, a wood-paneled dining room, a marble stairway, two elevators, a glassed-in breakfast room and a gown closet. She filled her mansion’s 54 rooms with Louis XVI furniture, Beauvais tapestries, Sèvres porcelains, Aubusson rugs, antique lace and paintings by Thomas Gainsborough.