Originally designed by General John T. Thompson in 1917-19, the M1 is a simplification of his original design, to make it easier to mass produce for World War II; later the M1A1 was developed to simplify the design even further. It was first issued to the troops in 1943. The M1A1 version has simpler blowback design than the original recoil-operated Thompson, and can only use 30-round "stick" magazines. It fires the .45ACP round, at a rate of about 700 rounds per minute. later the M1A1 has been developed to simplify the design even further.
It was used by the U.S. Military in all theaters of WWII, the Korean conflict and even saw action in the Vietnam War, before it was declared obsolete in the mid-70s.
It can regularly be retrieved from fallen enemies, particularly bank robbers and other more organized criminals.
- During one of Cole Phelps' flash-backs, some of the Japanese soldiers can be seen wielding M1 Thompsons, possibly taken from fallen American soldiers, though the game may have swapped the Japanese guns with Thompsons to save modeling time for guns not used in game.
- This weapon is used by Jack Kelso during the war, as was typical for Marine sergeants.
- In-game, this weapon features 30-round stick magazine, but its capacity in-game is only 20 and 30 in the PC version.
- An older version of the Thompson, called "Chicago Piano Gun", can be downloaded to the game.
- An explanation for the effectiveness of the Thompson M1 is the caliber of round it fires, 45 ACP, the same as the one used by Cole Phelps' sidearm as a Detective, the M1911A1 (or Star Model P), also a .45 ACP.
- Many were sold as Army surplus to police departments, some of which continue to use them to the present day.
- It has good damage, range, accuracy and rate of fire, and can usually kill an enemy in 2-4 hits, depending on the location of the hit.
- This weapon is more effective while wearing the Chicago Lightning outfit.