At age 16 he began attendance at the American Literary, Scientific and Military Academy, now called Norwich University, alternating semesters with work for his employer in Quechee, Vermont to pay his tuition.

[3][4] Ransom volunteered for the militia in the 1830s, and advanced through the ranks from captain to major general, gaining experience as a drill master and commander of units from company to brigade.

[5][6][7][8] In the 1830s and 1840s Ransom was part of a group of individuals interested in reorganizing and revitalizing the state militias, which had become increasingly dormant in the years following the War of 1812.

This group, including Franklin Pierce, Alden Partridge, Alonzo Jackman, and Frederic Williams Hopkins, held seminars to discuss tactics, strategy and recruiting, and carry out maneuvers and drills in an effort to improve unit readiness.

[12][13][14] Ransom volunteered to serve in the Mexican War and, on February 16, 1847, was appointed as the Major of the newly formed 9th Infantry Regiment under the command of Colonel Franklin Pierce.

[20] He was remembered by Richard Coulter Drum, a member of the 9th Infantry who participated in the battle and went on to serve as Adjutant General of the United States Army, as "by all odds the most brilliant man under fire I have ever seen.

[31] Thomas Edwin Greenfield Ransom served as a brevet major general in the Union Army during American Civil War and died in service on October 29, 1864.