He worked as the supervising engineer on the building and repair of many fortifications across the country.

[4] Cullum was appointed to the United States Army Corps of Engineers as a brevet second lieutenant on 1 July 1833.

He also supervised a number of other projects on the East Coast from 1840 to 1864, including the repairs of sea walls at Deer, Lovells and Rainsford islands; the construction of Forts Warren, Independence, Winthrop and Sumter; the building of cadet barracks at West Point; and the construction of the United States Assay building.

[3] Cullum was an instructor of practical military engineering at West Point from 1848 to 1851.

He took two years leave of absence from 1850 to 1852 for health reasons, and traveled throughout Europe, Asia, Africa and the West Indies while recuperating.

[6] Cullum published Description of a System of Military Bridges, With India-Rubber Pontons in 1849.

He served as an aide-de-camp to General Winfield Scott from 1 April 1861 to 1 November 1861.

The army was unprepared to handle this many prisoners and scrambled to find places to house them.

[3] For the rest of the Civil War, Cullum inspected or built defenses at: Cairo, Illinois; Bird's Point, Missouri; Fort Holt, Kentucky; Columbus, Kentucky; Island Number Ten; New Madrid, Missouri; Corinth, Mississippi; Harpers Ferry, West Virginia; Winchester, Virginia; Martinsburg, West Virginia; Boston Harbor; Nashville, Tennessee; the Potomac aqueduct; Baltimore; and Washington, D.C.

He served on boards concerning Timby's Revolving Iron Tower, proposed military bridges, and Army Corps of Engineers officers being considered for promotion.

In 1862, Cullum was appointed Chief Engineer of Halleck's armies in the Department of the Missouri.

[7] Cullum was vice-president of the American Geographical Society, president of the Geographical Library Society of New York and a member of the board of managers of the Association for Improving the Condition of the Poor.