He was born in Lancaster, Ohio. He was the older brother of John Sherman, the Senator who sponsored the Sherman Antitrust Act. Sherman's father, Judge Charles Sherman, died when Tecumseh was nine years old. The boy was informally "adopted" by a Lancaster neighbor, attorney Thomas Ewing, who served as a U.S. Senator from Ohio and as U.S. Secretary of the Interior. Ewing secured Sherman's apppointment to the United States Military Academy at West Point, from which he graduated sixth in his class of 1840. He entered the army as a second lieutenant and served through the end of the Mexican American War, during which he was stationed in California.
He resigned his military commission and became president of a bank in San Francisco. The bank failed in a financial panic in 1857. Sherman accept a job offered to him by two of his Southern army friends, P.G.T. Beauregard and Braxton Bragg, as the first president of Louisiana Military Seminary. He served in that post until the start of the American Civil War, when he resigned and went back north. It is an especially ironic that Louisiana Military Seminary later became Louisiana State University. On hearing of South Carolina's secession, he prophetically observed to a southern friend before going north to serve in the Union army:
Southerners most hate that the first president of what is now one of the most prestigious and beloved Southern universities was a Yankee general . Beauregard and Bragg both became Confederate generals during the Civil War. Sherman accepted a commission as a colonel in the U.S. Army and was one of very few Union officers to distinguish himself at the First Battle of Bull Run.
He was made a brigadier general and put in command of a military department in Louisville, Kentucky. During his time in Louisville, Sherman went through a personal crisis that has been described as a "nervous breakdown" or "insanity". Without question, he was likely working too hard, drinking and smoking too much as well. He suffered some sort of collapse which made it necessary for him to go home to Ohio to recuperate. Still, just six months later he was a brilliant and brave major general serving under Ulysses S. Grant at the April 1862 Battle of Shiloh. He suffered two slight wounds during that two-day battle in west Tennessee and had four horses shot from under him.
Sherman developed close personal ties to Grant during the two years they served together. At one point, not long after the Battle of Shiloh, Sherman persuaded Grant, who was being badly treated by his commander, General H. W. Halleck, to not resign from the army. The careers of both officers ascended considerably after that time. They shared in the glory of conquering Vicksburg in July 1863 and at the Battle of Lookout Mountain in Chattanooga. In later years Sherman said simply, "I took care of Grant when he was drunk, and he took care of me when I was crazy."
When Lincoln called Grant east in the spring of 1864 to take command of all Union armies, Grant appointed Sherman his successor as commander of the western theatre of the war. His siege and capture of Atlanta, Georgia and subsequent March to the Sea from Atlanta to Savannah in the autumn of 1864 sealed Sherman's position as one of the great Union heroes of the Civil War. When Grant became president in 1869, Sherman became the top general in the U.S. Army and served in that post until his retirement.
He led brutal campaigns against the natives. As with the Southerners, he tried to destroy their will to fight, not only by killing their soldiers, but by destroying the resources they needed to survive. Sherman believed that the natives were in the way of progress and might need to be exterminated, but he spoke out against government agents who treated the natives unfairly reservations.
In 1875 Sherman published his two-volume memoirs, a minor classic, marked by a forceful, lucid style, and the strong opinions for which Sherman has become famous.
Sherman retired from the army in 1884, and lived most of the rest of his life in New York City. He was devoted to the theatre and was much in demand as a colorful speaker at dinners and banquets in New York, among others.