Carnegie co-founded his first steel company near Pittsburgh in the early 1870s. He built a steel empire by owning factories, raw materials, and transportation infrastructure over the following few decades, maximizing revenues and eliminating inefficiencies. His major holdings formed Carnegie Steel Corporation in 1892.

The violent 1892 Homestead Strike at his Homestead, Pennsylvania, steel factory tarnished the steel magnate's reputation as a protector of workers. Carnegie Steel general manager Henry Clay Frick (1848-1919), anxious to dissolve the union, shut workers out of the plant when they protested wage reduction. Carnegie supported Frick, who hired 300 Pinkerton armed guards to secure the factory while on vacation in Scotland. At least 10 employees were killed in a Pinkerton-strike clash. The state militia took over the town, union leaders were jailed, and Frick hired new factory workers. The union lost the five-month strike. For 40 years, Pittsburgh steel mill workforce was decimated. John Pierpont Morgan (1837-1913) bought Carnegie Steel for $480 million in 1901, making Andrew Carnegie one of the world's richest men. Morgan formed U.S. Steel, the first billion-dollar organization, by merging Carnegie Steel with other steel companies that year.

After selling his steel company, Carnegie, who was 5'3", retired and dedicated himself to philanthropy. In 1889's "The Gospel of Wealth," he said that the rich have "a moral imperative to divide [their money] in ways that promote the welfare and happiness of the common man." “The man who dies so rich dies disgraced,” Carnegie added.

Carnegie handed up most of his riches, $350 million (billions in today's USD). He established more than 2,500 public libraries worldwide, donated more than 7,600 organs to churches worldwide, and endowed organizations (many still in existence) committed to science, education, world peace, and other causes.

Carnegie Hall, a famed New York City concert institution that founded in 1891, received $1.1 million from him. His donations established Carnegie Mellon University, Carnegie Institute for Science, and the Carnegie Foundation. He invested the most in public libraries in American history.

Carnegie's mother stayed with him until her 1886 death. In the next year, the 51-year-old industrial baron married Louise Whitfield (1857-1946), the daughter of a New York City businessman. Margaret was their only child (1897-1990). Skibo Castle, on 28,000 acres in Scotland, was the Carnegies' summer home and Manhattan house.

Shadowbrook, Carnegie's Lenox, Massachusetts house, was where he died at 83 on August 11, 1919. Sleepy Hollow Cemetery in North Tarrytown, New York, buried him.