George was born in 1826 to Dr. George and Elizabeth McClellan. George's academic ability became apparent at a young age and in 1839 he was able to begin attending Pennsylvania State University. Despite studying to be a lawyer he suddenly changed his mind and in 1841 applied to attend the US Military Academy.
At 14 years old he was accepted into the academy. His tutors noticed his extreme analytical mind and study with recent military history. Due to his age McClellan was unable to pass out after the usual two year course and had to spend an extra year in the academy. In 1844 he passed out first in his class with the rank of lieutenant.
McClellan was posted to the corps of engineers and helped plan the building of several land forts on the Virginia coast. In 1845, following the war with Mexico, McClellan was posted to California to an infantry regiment and participated in the battles of Los Nomas, Cartigua and Pestos. McClellan was promoted to Captain and put on the staff of General Scott.
In this position McClellan was responsible for supply and logistical activities. He remained in this post until 1847 when he was posted to the garrison at fort Middleton, Texas. During this posting McClellan befriended many southern officers, many future confederate generals.
He remained at the fort until 1852, when he was promoted to Major and sent as US military attache to the Paris embassy. McClellan used this chance to study the French army and its tactics.
In 1855 he was recalled and made a Colonel in charge of a regiment of militia who were trying to keep the peace in the western territories and prevent pro and anti-slavery conflicts. McClellan was in charge of the so called battle of Chego hill, where his forces protected a group of northern settlers from a southern mob.
By 1861 McClellan was a brigadier commanding the 3rd infantry division, soon to become the army of New York. On January 21st McClellan was promoted to Major General and was responsible for organising the forces in New York to face the confederates. However after in February he was given command of the army of Washington. He led the army during the Virginia campaign, and after defeating Lee's first attempt to besiege Washington, was hailed as a hero and replaced Winfield Scott as commander of the army.
McClellan spent his first year as commander organising the armies, and apart from the 2nd Fredericksburg in Autumn 1861 he did not see frontline action again. He had somewhat icy relations with President Lincoln, and after several major disagreements over the conduct of the war McClellan was sacked as commander in chief in 1864.
Within weeks McClellan announced his intention to run against Lincoln for the presidency that November. McClellan argued that Lincoln was making a series of U-turns about frontline fighting, and if any other man were in charge the war would have been "finished before it began". He advocated a total war strategy, saying it was the only way to restore the union. Despite major opposition from copperhead peace democrats he was able to get the democratic nomination that August, with fellow pro war "McClellanite" Andrew Johnson as his running mate. He went on to win the election in November.
He stuck to his promise of total war, launching a spring and summer offensives that year, and succeeded in breaking the siege of Washington in December 1865. In conjunction with General Grant, his successor as Commanding General, he was able to bring about Victory in April 1867, but at a price. Nearly 2 million Americans were killed during the war, 1 million during McClellan's presidency. The sheer loss of life, and the Republican controlled congress blocking almost all McClellan's legislation, made him unpopular. He was refused renomination by the democrats in 1868, preferring secretary of state Horatio Seymour (who went on to lose the election to Grant).
McClellan spent the next decade writing his war memoirs and travelling abroad. In 1878 he was persuaded to run as governor of New Jersey, and won. He declined a second term, and retired to his house in central New York City.
He died in 1885, a deeply devisive and controversial man, but following more favourable biographies of him written in the 1910's he has been viewed more favourably. In 1926 the McClellan-Lincoln memorial was unveiled by the sons of the respective presidents as a national memorial to the Union. In modern times he is one of the most popular democratic presidents, with his predecessor being largely forgotten.