1861
Carnegie works for Union Army.
After Confederate mobs in Maryland destroy railroad lines, Carnegie assists Thomas Scott in supervising repairs. While working on the railroad, Carnegie notices that telegraph lines have also been cut and stops to repair them. When Carnegie arrives in Washington, he joins Scott in organizing the railroad and telegraph lines to Virginia.

1861
Carnegie invests $11,000 in an oil company.
Using money from his investment in the Woodruff Sleeping Car Company, Carnegie invests $11,000 in an oil company in Titusville, Pennsylvania. He receives a return of $17,868 after only one year.

1862
Carnegie travels to Dunfermline.

1863
Carnegie's income is $42,000.
About half of Carnegie's salary comes from his investment in oil, and only $2400 from his salary at the railroad. Additional investments in the Piper and Schiffler Company, the Adams Express Company, and the Central Transportation Company contribute over $13,000.

1864
Carnegie drafted into army.
Carnegie is drafted into the Union Army. His options include paying the federal government $300 or finding a suitable replacement. Carnegie feels he has done his patriotic duty by supervising telegraph communications, and decides to pay a replacement $850 to serve in his place.

1865
Carnegie founds Keystone Bridge Company.
Carnegie and several associates reorganize the Piper and Schiffler Company into the Keystone Bridge Company. They envision building bridges with iron, rather than wood, to make the bridges more durable. Tom Scott loans Carnegie half of the $80,000 he needs for his investment.

1867
Carnegie founds Keystone Telegraph Company.
Carnegie establishes the Keystone Telegraph Company with several associates from the railroad. The company receives permission from the Pennsylvania Railroad to string telegraph wire across the railroad's poles, which stretch across the entire state. This is such a valuable asset that Keystone is able to merge almost immediately with the Pacific and Atlantic Telegraph Company, allowing Keystone's investors to triple their return.

1868
Carnegie pledges to resign from business at age 35.
Carnegie writes himself a letter which outlines his plans for the future. He determines to resign from business at age 35 and live on an income of $50,000 per year, devoting the remainder of his money to philanthropic causes, and most of his time to his education.