Young Henry detested the thought of an agricultural life and ran away from home at an early age to be raised by an uncle, David Wager of Utica.
He attended Hudson Academy and Union College, then the United States Military Academy. He became a favorite of military theorist Dennis Hart Mahan and was allowed to teach classes while still a cadet. He graduated in 1839, third in his class of 31 cadets, as a second lieutenant of engineers.
After spending a few years improving the defenses of New York Harbor, he wrote a report for the United States Senate on seacoast defenses, Report on the Means of National Defence, which pleased General Winfield Scott, who rewarded Halleck with a trip to Europe in 1844 to study European fortifications and the French military.
Returning home a first lieutenant, Halleck gave a series of twelve lectures at the Lowell Institute in Boston that were subsequently published in 1846 as Elements of Military Art and Science. His scholarly pursuits earned him the (later derogatory) nickname "Old Brains."
During the Mexican-American War, Halleck was assigned to duty in California. He spent several months in California constructing fortifications, then was first exposed to combat on November 11, 1847, during Shubrick's capture of the port of Mazatlán.
He was awarded a brevet promotion to captain in 1847 for his "gallant and meritorious service" in California and Mexico. He was transferred north to serve under General Bennet Riley, the governor general of the California Territory. Halleck was soon appointed military secretary of state, a position which made him the governor's representative at the 1849 convention in Monterey where the California state constitution was written. Halleck became one of the principal authors of the document. He was nominated during the convention to be one of two men to represent the new state in the United States Senate, but received only enough votes for third place.
During his political activities, he found time to join a law firm in San Francisco, Halleck, Peachy & Billings, which became so successful that he resigned his commission in 1854. He was a director of the Almaden Quicksilver (mercury) Company in San Jose, president of the Atlantic and Pacific Railroad, a builder in Monterey, and owner of the 30,000 acre (120 km²) Rancho Nicasio in Marin County, California. But he remained involved in military affairs and by early 1861 he was a major general of the California Militia.
Phi Beta Kappa
In 1855 he married Elizabeth Hamilton, granddaughter of Alexander Hamilton and sister of Union general Schuyler Hamilton. Their only child, Henry Wager Halleck, Jr., was born in 1856, and died in 1882.