With the start of the American Revolution, Scammell became a major in the 2nd New Hampshire Regiment, which was in Sullivan's Brigade, and after the Siege of Boston was sent with them to reinforce the Continental Army units in the Invasion of Canada.

Sullivan's force returned to Fort Ticonderoga by mid July 1776, and by August Scammell had been assigned as Aide-de-Camp to Sullvan.

But in September he was ordered to Assist Col George Reid in New York City and fought at the Battle of Long Island.

The recruiting of the 3rd NH Regiment was completed in June 1777, and within two weeks of men mustering for the first time at Fort Ticonderoga did Major General Arthur St. Clair order its evacuation.

Just two days after Burgoyne's surrender the regiment moved to winter quarters at Valley Forge where Scammell was appointed adjutant general of the Continental Army by Gen. George Washington.

The result was a letter of Nov 16, 1780 to Washington requesting permission to resign his post and take command of a regiment of the line.

[9] A monument was erected in Williamsburg to Scammell but may never have been engraved the following inscription:[10] Alexander Scammell, Adjutant-General of the American armies, and the Colonel of the First Regiment of New Hampshire, while he commanded a chosen corps of Light Infantry at the successful siege of Yorktown, in Virginia, was in gallant performance of his duty as field officer of the day, unfortunately captured, and afterwards insidiously wounded of which wound he expired at Williamsburgh, October 1781 Anno Ætatis.

What though no friend could ward thine early fall,Nor guardian angels turn the treacherous ball;Bless'd shade, be soothed!

A commemorative engraving, entitled "Death of Col. Scammell at the Siege of Yorktown", by Alonzo Chappel, was published by Johnson Fry & Company, NY, in 1859.

Scammell had the rare ability to lead and inspire loyalty as evinced by one of his captains, who at the Siege of Yorktown shortly after the taking of Redoubt #10, sacrificed his own honor by threatening the life of the captured Major Campbell to avenge his favorite (Col Scammell); Alexander Hamilton, who commanded the American assault, interceded saved Major Campbell.