William Boyd mustered as a second principal musician in the 1st Maine Cavalry in the summer of 1861 in Augusta.

In 1893, William L. Boyd, resident of Houlton, wrote an account of his capture of a Confederate Battle Flag in 1863:

"The Regt. was drawn up in the line on the pike to hold in check the advance of the Confederate column. An officer accompanied by a soldier rode to the crest of the hill and placed the flag. Adj. Boyd at once rode out from his command and met the party holding the flag. Sabers flashed and it was give and take. The Confederate was the better swordsman but Adj. Boyd was stronger and soon had his man to disadvantages. The flag was captured. It was done so quickly that no assistance could be furnished by either side. A general engagement took at once."

He received a number of promotions and was commissioned Captain of Co. L in 1865.