On September 1, 1775, the NC Provincial Congress appointed William Caswell as an Ensign under Capt. Simon Bright in the 2nd NC Regiment. This regiment was considered to be Provincial Troops at this point in time. On November 28, 1775, the Continental Congress accepted this unit on the Continental Line. Ensign William Caswell was with his company and regiment at the battle of Great Bridge, VA on December 9, 1775. It is possible that this company was also at the battle of Norfolk, VA on January 1, 1776, but this Author has found no definitive evidence thereof.

On April 17, 1776, the NC Provincial Congress appointed William Caswell as a Captain under Col. Edward Buncombe in the newly-created 5th NC Regiment, which was soon placed on the NC Continental Line. Capt. William Caswell led his company at the battles of Brandywine Creek, PA (9/11/1777) and Germantown, PA (10/4/1777). One source asserts that William Caswell resigned his commission on December 22, 1777. Other sources indicate that he was sent home in January of 1778 to recruit new troops for his regiment. The 5th NC Regiment was disbanded on June 1, 1778, but was recreated in January of 1779, without Capt. William Caswell.

In the 3Q of 1778, William Caswell is identified as second Colonel, alongside Col. Benjamin Exum, in the Dobbs County Regiment of Militia, replacing Col. James Glasgow, who had recently resigned his commission. Col. William Caswell led the Dobbs County Regiment of Militia under Brig. Gen. William Bryan and Maj. Gen. John Ashe, Sr. in the Purrysburg, SC expedition of late 1778/early 1779, and all were at the battle of Briar Creek, GA on March 3, 1779.

On May 9, 1779, the NC General Assembly appointed William Caswell as Brigadier General over the New Bern District Brigade of Militia, replacing Brig. Gen. William Bryan, who had resigned on April 27, 1779. Available records indicate that he retained this position until the end of the war in 1783.

Brig. Gen. William Caswell assembled the New Bern District Brigade of Militia and marched them towards Charlestown, SC in early May of 1780. While approaching the city, he learned of its surrender and he joined Virginia Col. Abraham Burford in their retreat. At Camden, SC, the two forces split up, and Brig. Gen. William Caswell went to Cross Creek - Col. Buford was surprised by the British Lt. Col. Banastre Tarleton at the infamous battle at the Waxhaws on May 29, 1780.

One source asserts that Brig. Gen. William Caswell led the New Bern District Brigade of Militia at the battle of Camden, SC on August 16, 1780 under his father Maj. Gen. Richard Caswell and the Continental Maj. Gen. Horatio Gates, but this Author has found no definitive evidence thereof. Three other North Carolina Brigadier Generals are known to have participated in the battle of Camden on that fateful day - Isaac Gregory, John Butler, and Griffith Rutherford, who was captured. Most records indicate that Col. Benjamin Exum led the many small detachments of the New Bern District Brigade of Militia at the battle of Camden, SC on August 16, 1780, while Brig. Gen. William Caswell was at home in Dobbs County.

On February 7, 1781, the NC General Assembly appointed James Armstrong as Brigadier General over the New Bern District Brigade of Militia, somehow thinking (unclear how) that William Caswell had resigned. However, they soon realized that William Caswell had not resigned and on February 10, 1781, he was re-instated by the NC General Assembly.

Brig. Gen. William Caswell led his brigade and other units at the battle of Rockfish Creek on August 2, 1781. As the occupying commandant of the British contingent in Wilmington, Maj. James H. Craig, marched to and from New Bern in August of 1781, Brig. Gen. William Caswell was instructed to keep at a safe distance and not to engage the enemy - at this point in time, there was a serious shortage of ammunition and the Patriots could only watch as Maj. Craig destroyed plantations and homes in New Bern as well as on his march through other counties.

William Caswell, the son of Richard Caswell and Mary McIlwaine, was born on September 24, 1754 in Johnston County (what soon became Dobbs County), and died on January 6, 1785 in Dobbs County, NC. He married Gathra McIlwaine on December 22, 1782, and they had one known son - Richard William.

In 1774, William Caswell was the Register of Deeds for Dobbs County. He was elected as a representative for Dobbs County in the NC House of Commons in 1779-1782, and again in 1784.