At the end of the war, Carnegie resigned from the Pennsylvania Railroad company, having begun a career as an investor and speculator. During the depression of 1873, he invested heavily in steel, and was able to raise the quality of steel while reducing its price by using technological innovations such as the Bessemer process. Gradually, he created a vertical monopoly in the steel industry by obtaining control over every level involved in steel production, from raw materials, transportation and manufacturing to distribution and finance. By 1897, he controlled almost the entire steel industry in the United States.
In 1901, Carnegie Steel merged with US Steel to become the largest company in existence at the time. He left the firm that same year, and devoted the rest of his life to philanthropic efforts. He created several charitable funds, such as the Carnegie Foundation for the Advancement of Teaching and the Carnegie Endowment for International Peace. He donated a substantial art collection to New York’s Metropolitan Museum of Art, and helped finance many public libraries. His investments He had once written that "the man who dies thus rich, dies disgraced." By the time of his death on August 11, 1919, in Lenox, Massachusetts, he had managed to give away all his money to the "benefaction of all mankind."