Designed many now-demolished landmarks, such as the first office to use elevators, the eight-story Equitable Life Assurance Society. For a time, his 20-story New York World Building was New York City's tallest building.
He was taught by Richard Morris Hunt from 1858 until 1860. He had previously earned a degree in civil engineering from New York University's Scientific School in 1858. He formed a partnership in 1860 with Charles D. Gambrill, but left for a time to fight in the Civil War.
His Western Union Telegraph building in New York was the first office building to rise to 10 stories.
He married Alice Stone in 1867.
He was under Ulysses S. Grant's command during his service in the Civil War.