Mestrezat served in several different regiments during the Spanish American War. The first noted was the 1st Regiment of West Virginia Volunteers. He served as the Chief Musician for about one year, then went on to serve as the Chief Musician of the 30th Regiment of the United States.
Mestrezat was honorably discharged in 1901 on the grounds of being mustered out. His character upon discharge was excellent, and his service was described as honest and faithful. However, Mestrezat's discharge was short lived as he reenlisted and was sent overseas for the Philippine Insurrection, where he served again as Chief Musician. During this time, he wrote a series of letters back to Morgantown to the newspaper The New Dominion. The letters were entitled From Mestrezat and covered everything from what it was like getting shot at, to the rations they were being served.
Walter A. Mestrezat is also credited with being the first director of what is now the Pride of West Virginia Mountaineer Marching Band. The band started out as an eight male ROTC band. The band's sum came to eleven when non ROTC males were allowed to join, but did not receive the same benefits as the ROTC members. Unhappy with this, the men broke off and formed a rebel band. The newly formed band had received permission from Mestrezat and the Athletic Director to perform at halftime shows at football games, but the University squelched this by stating they must be an official student organization. The men of the rebel band formed what is now the Omicron chapter of Kappa Kappa Psi, the national honorary band fraternity. The two bands merged into one band a few years later.