Early life and career
George Brinton McClellan was born in
Philadelphia, the son of a prominent surgeon, Dr.
George McClellan, the founder of
Jefferson Medical College.
 His father's family was of
Ulster Scots heritage.
 His mother was Elizabeth Sophia Steinmetz Brinton McClellan (1800–1889), daughter of a leading Pennsylvania family, a woman noted for her "considerable grace and refinement".
 The couple had five children: a daughter, Frederica; then three sons, John, George, and Arthur; and finally a second daughter, Mary. McClellan was the great-grandson of
Revolutionary War general
Samuel McClellan, of
Woodstock, Connecticut. He attended the
University of Pennsylvania in 1840 at age 13, resigning himself to the study of law. After two years, he changed his goal to military service. With the assistance of his father's letter to
John Tyler, young George was accepted at the
United States Military Academy in 1842, the academy having waived its normal minimum age of 16.
At West Point, he was an energetic and ambitious cadet, deeply interested in the teachings of
Dennis Hart Mahan and the theoretical strategic principles of
Antoine-Henri Jomini. His closest friends were aristocratic Southerners such as James Stuart,
Cadmus Wilcox, and
A. P. Hill. These associations gave McClellan what he considered to be an appreciation of the Southern mind and an understanding of the political and military implications of the sectional differences in the United States that led to the Civil War.
 He graduated in 1846, second in his class of 59 cadets, losing the top position to
Charles Seaforth Stewart only because of poor drawing skills.
 He was commissioned a
second lieutenant in the U.S. Army
Corps of Engineers.
McClellan's first assignment was with a company of engineers formed at West Point, but he quickly received orders to sail for the
Mexican–American War. He arrived near the mouth of the
Rio Grande in October 1846, well prepared for action with a double-barreled shotgun, two pistols, a saber, a dress sword, and a
Bowie knife. He complained that he had arrived too late to take any part in the American victory at
Monterrey in September. During a temporary armistice in which the forces of Gen.
Zachary Taylor awaited action, McClellan was stricken with
malaria, which kept him in the hospital for nearly a month. The malaria would recur in later years—he called it his "Mexican disease."
 He served as an engineering officer during the war, was frequently subject to enemy fire, and was appointed a brevet
first lieutenant for his services at
Churubusco and to
captain for his service at
 He performed reconnaissance missions for
Winfield Scott, a close friend of McClellan's father.
McClellan's experiences in the war would shape his military and political life. He learned that flanking movements (used by Scott at
Cerro Gordo) are often better than frontal assaults, and the value of siege operations (
Veracruz). He witnessed Scott's success in balancing political with military affairs, and his good relations with the civil population as he invaded, enforcing strict discipline on his soldiers to minimize damage to property. McClellan also developed a disdain for volunteer soldiers and officers, particularly politicians who cared nothing for discipline and training.
McClellan returned to West Point to command his engineering company, which was attached to the academy for the purpose of training cadets in engineering activities. He chafed at the boredom of peacetime garrison service, although he greatly enjoyed the social life. In June 1851 he was ordered to
Fort Delaware, a masonry work under construction on an island in the
Delaware River, 40 miles (64 km) downriver from Philadelphia. In March 1852 he was ordered to report to Capt.
Randolph B. Marcy at
Arkansas, to serve as second-in-command on an expedition to discover the sources of the
Red River. By June the expedition reached the source of the north fork of the river and Marcy named a small tributary McClellan's Creek. Upon their return to civilization on July 28, they were astonished to find that they had been given up for dead. A sensational story had reached the press that the expedition had been ambushed by 2,000
Comanches and killed to the last man. McClellan blamed the story on "a set of scoundrels, who seek to keep up agitation on the frontier in order to get employment from the Govt. in one way or other."
In the fall of 1852, McClellan published a manual on bayonet tactics that he had translated from the original French. He also received an assignment to the Department of Texas, with orders to perform a survey of Texas rivers and harbors. In 1853 he participated in the
Pacific Railroad surveys, ordered by
Secretary of War
Jefferson Davis, to select an appropriate route for the planned
transcontinental railroad. McClellan surveyed the western portion of the northern corridor along the 47th and 49th parallels from
St. Paul to the
Puget Sound. In so doing he demonstrated a tendency for insubordination toward senior political figures.
Isaac Stevens, governor of the
Washington Territory, became dissatisfied with McClellan's performance in scouting passes across the
Cascade Range. McClellan selected Yakima Pass without a thorough reconnaissance and refused the governor's order to lead a party through it in winter conditions, relying on faulty intelligence about the depth of snow pack in that area. In so doing, he missed three greatly superior passes in the near vicinity, which would be the ones eventually used for railroads and interstate highways. The governor ordered McClellan to turn over his expedition logbooks, but McClellan steadfastly refused, most likely because of embarrassing personal comments that he had made throughout.
Returning to the East, McClellan began courting his future wife, Mary Ellen Marcy (1836–1915), the daughter of his former commander. Ellen, or Nelly, refused McClellan's first proposal of marriage, one of nine that she received from a variety of suitors, including his West Point friend, A. P. Hill. Ellen accepted Hill's proposal in 1856, but her family did not approve and he withdrew.
In June 1854, McClellan was sent on a secret reconnaissance mission to Santo Domingo at the behest of Jefferson Davis. McClellan assessed local defensive capabilities for the secretary. (The information was not used until 1870, when President
Ulysses S. Grant unsuccessfully attempted to annex the
Dominican Republic.) Davis was beginning to treat McClellan almost as a protégé, and his next assignment was to assess the logistical readiness of various railroads in the United States, once again with an eye toward planning for the transcontinental railroad.
 In March 1855, McClellan was promoted to captain and assigned to the 1st U.S. Cavalry regiment.
Because of his political connections and his mastery of French, McClellan received the assignment to be an official observer of the European armies in the
Crimean War in 1855. Traveling widely, and interacting with the highest military commands and royal families, McClellan observed the siege of
Sevastopol. Upon his return to the United States in 1856 he requested assignment in Philadelphia to prepare his report, which contained a critical analysis of the siege and a lengthy description of the organization of the European armies. He also wrote a manual on
cavalry tactics that was based on Russian cavalry regulations. Like other observers, though, McClellan did not appreciate the importance of the emergence of
rifled muskets in the Crimean War, and the fundamental changes in warfare tactics it would require.
The Army adopted McClellan's cavalry manual and also his design for a
saddle, dubbed the
McClellan Saddle, which he claimed to have seen used by
Prussia and Hungary. It became standard issue for as long as the U.S. horse cavalry existed and is still used for ceremonies.
George B. McClellan and Mary Ellen Marcy (Nelly) McClellan
McClellan resigned his commission January 16, 1857, and, capitalizing on his experience with railroad assessment, became chief engineer and vice president of the
Illinois Central Railroad, and then president of the
Ohio and Mississippi Railroad in 1860. He performed well in both jobs, expanding the Illinois Central toward
New Orleans and helping the Ohio and Mississippi recover from the
Panic of 1857. Despite his successes and lucrative salary ($10,000 per year), he was frustrated with civilian employment and continued to study classical military strategy assiduously. During the
Utah War against the
Mormons, he considered rejoining the Army. He also considered service as a
filibuster in support of
Benito Juárez in Mexico.
Before the outbreak of the Civil War, McClellan became active in politics, supporting the presidential campaign of
Stephen A. Douglas in the
1860 election. He claimed to have defeated an attempt at vote fraud by
Republicans by ordering the delay of a train that was carrying men to vote illegally in another county, enabling Douglas to win the county.
In October 1859 McClellan was able to resume his courtship of Mary Ellen, and they were married in
Calvary Church, New York City, on May 22, 1860.