After completing his schooling in New Jersey in 1865, Hardenbergh moved to New York to apprentice in the offices of the architect Detlef Lieneau. There, he was greatly influenced by Lieneau's Beaux-Arts style, and in 1870 he left to open his own architectural practice.
It was not until 1879, however, that Hardenbergh received his first major commission, the Vancorlear Hotel, built for Edward S. Clark. In 1880, Clark commissioned the young architect to design the Dakota, America's first luxury apartment house. Hardenbergh's future was secure. During the following years, he was engaged in numerous projects and designed such buildings as the Waldorf and Astor hotels in New York and the Copley Plaza Hotel in Boston. He was a proponent of the German-Renaissance Revival in America and was considered one of the leading architects of the day.
