George Brinton McClellan was born on December 3, 1826, in Philadelphia, Pennsylvania. McClellan was born to George McClellan, a Yale graduate who completed medical school at the University of Pennsylvania, and to Elizabeth S. Brinton. At five years of age, McClellan entered school. He attended private schools and a preparatory school before entering the University of Pennsylvania in 1840. After two years at the University of Pennsylvania, McClellan decided to enter West Point in 1843 at the astonishing age of 16. It is said that his decision may have been due to economic pressure, despite the fact his father was a distinguished doctor who was one of the founders of Jefferson Medical College.

At West Point, McClellan, or "Little Mac" as his classmates called him due to his short stature, proved to be an excellent student. McClellan excelled greatly in mathematics and became known as a "kind, social athlete," according to H.J. Eckenrode's book. Eckenrode also quotes General Dabney H. Maury, who was a classmate of McClellan at West Point, as saying, "A brighter kindlier, more gentle gentleman did not live than he." Maury also described McClellan's ability to learn Spanish, German, and a basic knowledge of the Russian language. After graduating second in his class, it was no surprise that McClellan, a student skilled in mathematics, chose the highly-regarded engineer branch. After graduating on June 30, 1846, he was assigned to a company at West Point as a 2nd Lieutenant, and he prepared for the Mexican War there. Later that year on September 24, his company left for the Texas-Mexican border.

Upon his arrival McClellan became sick, but recovered and moved along on an expedition that would eventually lead him to Cerro Gordo, the National Road in Mexico. On his mission to Mexico City, McClellan learned a lot about the army and how it functioned. McClellan encountered a great deal of fighting, which became fiercer as the Army approached Mexico City. During the Battle of Contreras, McClellan used his knowledge of engineering to help push the Mexicans back and was slightly wounded during battle. McClellan had fought in the Battles of Churubusco and Chapultepec and was promoted to the rank of Captain for his excellence. After entering Mexico City on September 14, 1847, McClellan returned to the United States in June 1848 with a great reputation.

After McClellan's return from Mexico, he remained at West Point for three years, although he had a strong desire to leave. In 1851, McClellan was sent as an assistant engineer to Fort Delaware, and then in January 1852, he was assigned to accompany Captain Randolph B. Maury in an expedition to explore the sources of the Red River in Arkansas. During his joyful journey he met his future wife Ellen Marcy. After this, McClellan was sent on other expeditions, including finding a path for the Transcontinental Railroad and determining the value of the harbor of Samana in San Domingo. In 1855, he changed his service branch in the Army, and he was promoted to captain in the First Cavalry Regiment. Following this, McClellan was one of three officers sent off to observe the European military. McClellan learned a lot by traveling all over Europe and Russia during the Crimean War. After a year of studying foreign tactics, McClellan returned to America in 1856, and he was then perhaps the best-trained and most intelligent officer in the Army. His information was very beneficial to the army, especially his introduction of the Hungarian saddle, which came to be known as the "McClellan saddle" in the United States.

In 1857, McClellan resigned from the Army and became chief engineer of the Illinois Central Railroad. In 1860, after settling down in Chicago, Illinois, McClellan became president of the eastern division of the Ohio and Mississippi Railroad. Because of his great success and his high paying job, Ellen Marcy, whom McClellan had known for some years, was encouraged by her family to marry him. On May 22, 1860, the successful engineer married Marcy in New York.

In 1861, the Civil War broke out, and McClellan, who was a strong Unionist, quickly accepted an appointment of Major General in command of the Ohio Volunteers on April 23. On May 13, he was given the command of the Department of Ohio, which included the states of Indiana, Ohio, and Illinois. McClellan efficiently trained his men, turning them into organized and disciplined soldiers. While commanding, McClellan was often in disagreement with army Commander-in-Chief, Winfield Scott. Scott refused McClellan's requests for artillery and cavalry and also did not allow him to make contact with General Patterson in command of the Union forces on the upper Potomac River. Due to McClellan's inability to assist General Patterson, Patterson lost the Battle of Manassas, also known as the Battle of Bull Run.

At the time, McClellan was the 2nd ranking officer in the Army and became increasingly disliked by the War Department due to his numerous requests for additional supplies and troops. Although they disliked him, the Department admired his work and awarded him control of Missouri. Soon after, McClellan sent troops to stop Confederate forces that were sent to suppress the idea of a Union revolution in Richmond, Virginia. McClellan's volunteer troops succeeded and gained control of western Virginia. Even the famous Confederate General and former ally, Robert E. Lee, could do nothing to push back McClellan's troops. Pleased with McClellan's victory, President Lincoln informed McClellan that he was to also control the troops in Washington.

The troops in Washington were battered and demoralized, and McClellan did his best to re-train these men. Also, McClellan got rid of political appointees who were holding high military positions, but this action angered President Lincoln. Not only did McClellan upset Lincoln, but he also upset other Civil War authorities by constantly criticizing them and refusing to take part in a Virginia Campaign. McClellan soon gained control of the Army of the Potomac and was scrutinized by civil authorities for not taking the offensive. McClellan disagreed with these civil authorities, who were mostly radical Republicans on the Committee on the Conduct of War, because they believed that advancing to Manassas would lead to a Union victory in the war. Later, on October 12, McClellan's first child was born. On December 20, Lieutenant General Scott was forced to retire, making McClellan Commander of the Army. As January approached, McClellan came down with typhoid and did not fully return to office until January 13, 1862. In February, Lincoln, despite McClellan's objection, ordered the Army of the Potomac to advance to Manassas.

Unfortunately, McClellan, who continued to anger higher authorities, especially President Lincoln, was relieved of command of the Army. He was demoted to the control of the Potomac. After moving his forces to Fort Monroe, more and more Republicans took office and chose to demote the democratic officer even further to commander of only the Army of the Potomac. To make matters worse, much of his army was then reassigned elsewhere. With the remaining men, McClellan advanced to Yorktown and laid siege to the city, giving opposing General Albert Sidney Johnston time to reinforce and then evacuate his position. McClellan did not attack earlier because he believed it would have resulted in a loss, so instead he decided to take extra time and position siege weapons. Although McClellan took Yorktown, Lincoln continued to listen to Secretary of War Edwin M. Stanton, who had nothing but negative things to say about McClellan. Stanton convinced Lincoln that McClellan's non-ambitious strategies were costing the Union the war.

On May 31, McClellan pushed forward to Chickahominy, then to Fair Oaks, where the battle of the Seven Pines was fought. In the beginning, McClellan caused much damage to General Johnston, but Robert E. Lee eventually took command and helped the Confederate forces greatly. This event led to the Seven Days Battles, which consisted of six major battles from June 25 to July 1. These battles eventually led to a Union retreat, and the forces were then sent to reinforce General Pope in Central Virginia.

After this event, McClellan lost command of the Army of the Potomac, but then he regained it when he fought with General Pope, who was losing many battles. After gaining control of the Army of the Potomac, General Lee's forces crossed the Potomac River into Maryland during September. McClellan sent forces to oppose General Lee. Their forces collided in the Battle of South Mountain and the Battle of Antietam. Lee's forces, who had suffered huge losses in these two battles, retreated. McClellan pursued Lee across the Potomac River, but failed to destroy Lee's weakened army.

McClellan encountered the same mistakes he made previously, and he did not command or succeed in the way the Committee on the Conduct of War and President Lincoln wanted him to. President Lincoln could not tolerate McClellan's refusal of orders or his choices to not always pursue the enemy. He believed McClellan was only hampering the war efforts. Lincoln made it very clear in his letters to McClellan that he did not approve of McClellan's attitude and was fed up with hearing McClellan always give reasons on why he should not attack or pursue.

Lincoln, along with the other war authorities, wanted to win the war. With a strong desire to progress, they were no longer willing to put up with McClellan's constant belief that the Union Army should take their time and use extra precaution. In addition, Lincoln was also fed up with McClellan always demanding more support. In an interview found in Spartacus, dated in March, 1863, Lincoln said:

"I directed McClellan peremptorily to move on Richmond. It was eleven days before he crossed his first man over the Potomac; it was eleven days after that before he crossed the last man. Thus he was twenty-two days in passing the river at a much easier and more practicable ford than that where Lee crossed his entire army between dark one night and daylight the next morning. That was the last grain of sand which broke the camel's back. I relieved McClellan at once."

After his failure, on November 8, Lincoln ordered the removal of the demanding general from active duty. Before his removal, Lincoln was quoted saying, "My dear McClellan: If you don't want to use the Army I should like to borrow it for a while." Lincoln replaced McClellan with Ambrose Burnside. McClellan was forced to stay in Trenton, New Jersey, and started building relations with the Democratic Party.

In 1864, McClellan ran as the Democratic Presidential candidate against the man who had relieved him of active duty. McClellan's opinion on the war was different from Lincoln's. He believed Lincoln was unable to handle the Civil War and thought that the President was going about the war all wrong. Also, as stated in Eckenrode's book, McClellan believed that the "individual liberties of American Citizens as well as the rights of states were endangered." Despite his growing belief that Lincoln was mishandling the war, McClellan was unable to win the presidential election.

After his loss, McClellan and his family sailed to Europe in 1865 and spent time traveling through different European countries. In November 1866, McClellan's son George was born in Europe. After a few years of travel, McClellan and his family sailed back to the United States in September 1868. Upon his return, McClellan worked on developing an experimental floating battery, and then from 1870 to 1872, McClellan made a wealthy living as engineer-in-chief of New York's Department of Docks. Soon after, McClellan moved on to become President of the Atlantic and Great Western Railway. In 1873, he started a consulting firm of engineers and accountants. In 1873, McClellan was nominated by the Democratic Party for the governorship of New Jersey. McClellan accepted the offer and won the election. He served as Governor from 1878 to 1881.

After his term as Governor, McClellan spent the remainder of his life traveling and writing. His memoirs were published after his death and were called McClellan's Own Story, which was a survey of his military career. On October 28, 1885, McClellan died suddenly of heart failure and was buried in Trenton, New Jersey. The General's most famous memoriam includes a bronze statue honoring his accomplishments in Washington D.C. and Fort McClellan in Alabama.