Born in Westfield in the Province of Massachusetts Bay, he attended the common schools, engaged in agricultural pursuits, and served in the French and Indian Wars for six years.

[4] He entered the Continental Army in May 1775 as lieutenant colonel and was commissioned Colonel of the 4th Massachusetts Regiment on October 6, 1776, serving throughout the Revolutionary War, including winter at Valley Forge, Pennsylvania, where he commanded the 4th Regiment of Massachusetts Continentals, under the overall command of General John Glover.

In this time local farmers and veterans of the war began to rebel after months of destitution and taxation they believed to be unfairly levied by the powers from Boston.

Shepard, then a major general in the state militia, called to duty the Fourth Division of the Massachusetts militia in 1786 and defended the Springfield Armory during what became known as Shays' Rebellion (after one of its principal leaders, Daniel Shays), ordering defenders of the arsenal to fire cannons at attacking the rebels at "waist height" with cannons filled with anti-personnel grape shot.

He kept in constant contact with Governor Bowdoin, Sam Adams, John Hancock, and General Benjamin Lincoln, who arrived in a blizzard from Boston just after the Springfield arsenal attack to pursue Shays and his men into the surrounding towns.