In 1900, then-Captain Lewis was sent by Adjutant General Henry Clarke Corbin to Europe to study that subject,[1] his report resulting in the re-armament of the field artillery.

By successive promotions, he rose to the rank of colonel in the Coast Artillery Corps in August 1913, and he was retired the next month for disability incurred in line of duty.

[2] In 1911, he refined an original machine gun design of Samuel Maclean and began active marketing of a type which came to be known simply as the "Lewis Gun", which was used in World War I by the Allied armies, the United States Navy, and the airplanes of the United States and Allies.

Lewis, already a wealthy man, declined the royalties—amounting to at least $1,200,000 ($35,557,320 in 2022 terms)—on guns made for the United States after it entered the war.

Lewis died from a myocardial infarction at the Lackawanna Railroad terminal in Hoboken, New Jersey, while waiting for a train to his home in Montclair.