The M1941 Johnson Light Machine Gun, also known as the Johnson and the Johnny gun, was an American recoil-operated light machine gun designed by Melvin Johnson in the late 1930s. It shared the same operating principle and many parts with his M1941 Johnson rifle and M1947 Johnson auto carbine.

The M1941 Johnson light machine gun was designed by a Boston lawyer and captain in the Marine Corps Reserve, Melvin Johnson Jr. His goal was to build a semi-automatic rifle that would outperform the M1 Garand the US Army had adopted. By late 1937, he had designed, built, and successfully tested both a semi-automatic rifle and a prototype light machine gun. Each shared a significant number of physical characteristics and common parts, and both operated on the principle of short recoil with a multi-lug rotating bolt (first used on the Fosbery Pump Shotgun). He took the parts of other guns, switching them out and creating the M1941 light machine gun.

Johnson's curved, single-column magazine attached to the left side of the receiver; company brochures list a 20-round magazine as standard. Additionally, the weapon could be loaded by stripper clip at the ejection port, or by single rounds fed into the breech. The rate of fire was adjustable, from 200 to 600 rounds per minute. Two versions were built: the M1941 with a wooden stock and a metal bipod, and the M1944 with a tubular steel butt and a wooden monopod.

The design intended the recoil forces to travel, along with the mass of the weapon's moving parts, in a direct line to the shoulder of the gunner. While this design minimized muzzle climb, the sights had to be placed higher above the bore.

The weapon has many parallels with the German FG 42 automatic rifle. Both feed from the left side, and both fire from an open bolt while in automatic, and a closed bolt while in semi-auto. Both weapons were awkward to carry loaded, with a side-mounted magazine; the Johnson had an especially lengthy single-column magazine, and this feature tended to unbalance the weapon. Despite these similarities, there is no evidence that either weapon had any effect on the design of the other. Both weapons attempted to solve similar problems, and adopted similar solutions.

Prototypes of semi-automatic rifles, 20-round magazine-fed,^{[citation needed]} based on the Johnson LMG were also produced. The M1947 Johnson auto carbine is an example. A belt fed variant, the T48 light machine gun also existed.

Johnson sold small quantities of the Johnson LMG to the U.S. Army and Marine Corps.^{[page needed]}

During World War II, Allied special forces demanded a more portable, lighter, more accurate automatic rifle that provided the equivalent stopping power of the American BAR. As a result, this machine gun was adapted as the BAR replacement for commandos operating behind Axis lines. The First Special Service Force commandos, raised jointly with men from both Canada and the United States (the famous Devil's Brigade), traded the Marine Corps 125 of the new Johnson light machine guns for plastic explosives. They were used in lieu of BARs, but as they wore out and were lost in combat they were replaced by BARs.