William Astor Sr. (1792-1875) was an American businessman and real estate mogul, known for his significant contributions to the Astor family fortune. He was the son of John Jacob Astor, one of the first multi-millionaires in the United States, and he expanded the family’s wealth through strategic investments in New York City real estate. Astor Sr. played a crucial role in the development of the city during the 19th century, acquiring valuable properties and helping to shape its urban landscape. His success laid the groundwork for future generations of the Astor family, solidifying their status as one of America's wealthiest families.