The 181st Infantry Regiment shares the distinction of being the oldest combat regiment currently organized in the United States Army. It is one of several National Guard units with colonial roots and campaign credit for the War of 1812. The regiment traces its history to 13 December 1636, when it was one of four colonial regiments of foot of colonial Massachusetts militia. It later served in the Continental Army during the American Revolution, with Union forces in the American Civil War, and as a federalized Massachusetts National Guard regiment with the U.S. Army during War with Spain, Mexican Border Campaign, World War I, and World War II. In 2006 Company A (Agawam) of the battalion deployed as a member of KFOR8 to Kosovo in support of Operation Joint Enterprise.

Most recently the 1st Battalion, 181st Infantry has served in Iraq, in New Orleans following Hurricane Katrina, and in Afghanistan. The only active element of the regiment is the 1st Battalion, 181st Infantry Regiment, which returned from a year of service in Afghanistan in July 2011. The 1st Battalion was mobilized in March 2017 for one year of service with the Multinational Force & Observers in the Sinai Peninsula of Egypt. Currently, the 1st Battalion is a part of the 44th Infantry Brigade Combat Team, a major formation of the New Jersey National Guard. It was reflagged from the 50th Infantry Brigade Combat Team in 2017.

The 181st Infantry Regiment traces its history to the organization of the North Regiment on 13 December 1636 in the Massachusetts Militia. It was formed from existing trained bands (local militia). It was redesignated on 7 September 1643 as the Middlesex Regiment. The unit was expanded 13 October 1680 to form the 1st (or Lower) Middlesex Regiment and the 2nd (or Upper) Middlesex Regiment, consisting of companies from Concord, Bedford, Sudbury, Marlborough, Chelmsford, Billerica, Groton, Acton, Lancaster, and Dunstable. The lineage of the 1st Middlesex Regiment separates here and is held by the 182nd Infantry Regiment (United States).

The soldiers of the 2nd Middlesex Regiment fought at the Battle of Lexington and Concord on 19 April 1775. The regiment was reorganized and entered the Massachusetts Army as elements of Prescott's Regiment, Thomas' Regiment, Bridge's Regiment, Nixon's Regiment, and Johnathan Brewer's Regiment of the Massachusetts Line.

The units were redesignated on 1 January 1776 as the 6th Continental Regiment, the 13th Massachusetts Regiment, and the 23rd Continental Regiment of the Continental Line, and fought in the following campaigns: the Battle of Lexington and Concord, the Siege of Boston(Bunker Hill), the Battle of Long Island, the Battle of Trenton, the Battle of Princeton, the Battle of Saratoga, and the Battle of Monmouth.

The regiment is additionally entitled to battle honors through the 104th Infantry (Hampshire Regiment) for Battle of Quebec 1775 and Rhode Island 1780.

The 181st Infantry is one of only nineteen Army National Guard units with campaign credit for the War of 1812. The Massachusetts militia was one of the largest, best equipped, and best trained of any of the state militias but support for the war in New England was lukewarm at best. As a consequence, no Massachusetts units were federalized until 1814, although as state units they were active in guarding the state's coastline. Only after Great Britain burned Washington City and generally increased its naval pressure on the East Coast did Massachusetts allow its militias to be mustered into federal service.

With the start of the Civil War, the 6th Massachusetts (Militia) was ordered into active service for the defense of Washington in April 1861. As it marched to the relief of the capital, it was attacked by a pro-southern mob in Baltimore in what would later be called the Pratt Street Riot. The regiment fought its way through, leaving four of its own dead on the streets of the city. On their arrival in Washington, they were greeted by President Lincoln, who shook Colonel Jones's hand and said, "Thank God, you have come" as theirs was the first armed and trained regiment to arrive in Washington. They slept that night in the Capitol building. The regiment was mustered into federal service on 22 April 1861 at Washington, D.C., for three months service, and served in the defense of Washington before being mustered out on 2 August 1861 at Boston.