At the instruction of the Secretary of War, Lewis Cass, he prepared A Digest of the Laws Relating to the Military Establishment of the United States.

[2] In 1840, he was a member of a commission sent by the Ordnance Board on a nine-month trip to Europe to visit arsenals and cannon foundries to report on the latest artillery improvements there.

[2] Mordecai became assistant inspector of arsenals in 1842 and was engaged in constructing and experimenting with ballistic pendulums and gunpowder, with the idea of measuring muzzle velocity.

[9] In 1855–57 he was sent, along with Major Richard Delafield and Captain George B. McClellan, as a member of a military commission to act as an observer during the Crimean War.

Instead, Mordecai emphasized marksmanship:[9] What mattered in a rifle, he believed, was its ability to allow a soldier to shoot his target at the longest distance possible and with the bare minimum of ammunition.

[13] With the advent of the Civil War in 1861, Mordecai was unwilling to fight either against the Confederacy or against his son, also called Alfred, who was serving in the Union Army.

[3] Rose notes that "Mordecai grieved for the North's abolitionist 'interference' in affairs below the Mason-Dixon Line, but he had 'no sympathy' with slavery and he was a good and faithful servant of the federal government.

After a brief stint in Mexico, where he worked as an assistant engineer for the Mexican Railway,[14] Mordecai served as secretary and treasurer of the Pennsylvania Canal Company from 1867 until his death.

It was an example and an inspiration for every other worker in the same field, and Mordecai was respected by all of them for his technical contributions no less than he was loved for his fineness of character, integrity, warmth and gentle humor.

"[16] According to The Jewish Press, Mordecai is best known for "introducing scientific methods into the development of pre-Civil War military munitions that contributed to America's becoming a nineteenth century world power.

The value of his work consisted in its accuracy, its systematic character, and its immediate utility, and still more in the subtle, potent way in which the spirit of it pervaded almost insensibly the entire corps.

Alexander Rose notes that[9] By preserving marksmanship as the quintessential American trait — a belief adopted by the nascent NRA from the 1870s onward — he can be counted as the spiritual father of such fine-shooting legends as the Springfield Model 1903 and the M1 Garand.