Simon Cameron (March 8, 1799 – June 26, 1889) was an American businessman and politician who was four times elected senator from Pennsylvania, and whose involvement in politics spanned over half a century. He served as United States Secretary of War under President Abraham Lincoln at the start of the American Civil War.

A native of Maytown, Pennsylvania, Cameron made a fortune in railways, canals, and banking. Initially a supporter of James Buchanan, whom he succeeded in the Senate when Buchanan became Secretary of State in 1845, Cameron broke with Buchanan and the Democratic Party by the 1850s. An opponent of slavery, Cameron briefly joined the Know Nothing Party before switching to the Republicans in 1856. He won election to another term in the Senate in 1857 and provided pivotal support to Lincoln at the 1860 Republican National Convention.

Lincoln appointed Cameron as his first Secretary of War. Cameron's wartime tenure was marked by allegations of corruption and lax management, and he was demoted to minister to Russia in January 1862. He remained there only briefly, and returned to the United States. There, he rebuilt his political machine in Pennsylvania, securing a third term in the Senate in 1867. After ten years in his third stretch in office, he resigned, arranging the election of his son, J. Donald Cameron, in his place. Cameron lived to the age of 90; his machine survived him by several decades, dominating Pennsylvania politics until the 1920s.

Cameron was born in Maytown, Pennsylvania, on March 8, 1799, the third of eight children of Charles Cameron and his wife Martha Pfautz Cameron. Charles Cameron's father, named Simon, had emigrated from Scotland in 1766 to the colonial-era Province of Pennsylvania. A farmer, he continued his trade in Lancaster County and fought with the Continental Army in the Revolutionary War. Martha Cameron was the granddaughter of Hans Michel Pfoutz, one of the first Palatine Germans to emigrate to the Thirteen Colonies.

Charles Cameron was a tailor and tavern keeper in Maytown, but was less than successful in those occupations. In 1808, he moved from Lancaster County north to Sunbury, in Northumberland County, but within two years was living in Lewisburg with his wife, but without his children. He died in January 1811, and his children then boarded with other families. Simon was sent to live with the family of Dr. Peter Grahl, a physician in Sunbury. The Grahls were childless and treated him like their son, and he expanded his rudimentary education in the libraries of Dr. Grahl and his neighbors. In Sunbury, he met and got to know Lorenzo Da Ponte, a librettist for Mozart and other composers. In December 1813, Simon visited Philadelphia with him.

Simon Cameron was in every respect a self-made man, but many contemporaries, including four presidents, considered the end product worthless.

Soon after his 17th birthday, Cameron apprenticed himself as a printer with Andrew Kennedy, publisher of the Sunbury and Northumberland Gazette and Republican Advertiser. In 1817, Kennedy, who had suffered financial troubles, released Cameron from his indentures, and he went to Harrisburg, where he indentured himself to James Peacock, publisher of the Pennsylvania Republican, the leading Pennsylvania newspaper outside of Philadelphia; after two years' apprenticeship, Cameron was made the newspaper's assistant editor.

Involvement with a newspaper in Harrisburg, the state capital, meant involvement in Pennsylvania politics; in 1842, Cameron said he had attended almost every session of the Pennsylvania General Assembly, the state legislature, since 1817. He met Samuel D. Ingham, the Secretary of the Commonwealth and proprietor of the Doylestown Messenger. Following the departure of its editor, he hired Cameron as his replacement in January 1821. Cameron held this position throughout the year, but the newspaper was not profitable and merged with another local paper, costing Cameron his job.