It is the standard sidearm of the USMC and CIA in the campaign. It can be seen first in "S.O.G." used by a Marine who is struck by an artillery shell as Hudson is transported to the bunker. This is when it is first available.
In Victor Charlie, the player uses the M1911 found on the floor of the Huey the mission starts out in to kill the Viet Cong who killed the pilots of said Huey. It remains with the player throughout the initial, stealthy part of the mission until Mason detonates the C4, where it is replaced by a Commando and a WA2000 (however, the Wii version retains the M1911 instead of the WA2000).
It later appears in the mission WMD. It is given to the player in the first building with a silencer attached, and can be swapped out for a Soviet weapon for use in the breaching of the next building. After the SR-71 sequence, it is replaced by a Crossbow.
The M1911 is a selectable secondary weapon in Black Ops. It is an unlocked pistol at level 1 along with the Makarov and ASP in the default classes. All three sidearms have the same damage and multipliers. What sets them apart is that the ASP has a higher max firecap and lower recoil while the Makarov has an extra bullet in the magazine. It takes 3 shots to kill an enemy at close range and 5 shots to kill an enemy at medium to long range. It also has moderate idle sway. In a way, the M1911 without any attachments is inferior to all other handguns. The ASP has the same amount of ammo, but a faster firing rate, reload, and draw time. The Makarov and CZ75 have a higher capacity, and the Python has higher damage. However, the M1911 with Extended Magazines gives it more ammunition than all other starting pistols, though at later levels it should be replaced by CZ75, as when the latter is used with Extended Mags, it has a 18 round capacity, superior to any other pistol. Many players consider the M1911 the worst pistol, instead taking the Python or CZ75.
This weapon is also used in the One in the Chamber Wager Match game mode as the only available weapon, along with the knife.
The M1911 is the starting weapon in all Zombie levels with 8 bullets loaded in the first magazine and 32 in reserve. It functions the same as in World at War. It is one of the weakest weapons, and is only good for the first few rounds. It is recommended to go for headshots, as it is easier on early rounds and it takes fewer shots. When upgraded via the Pack-a-Punch Machine it is renamed the Mustang & Sally (as seen on the Black Ops front cover.) The upgrade gives it the Dual Wield attachment, firing grenades instead of regular bullets at the cost of individual magazine size decreased to 6. It can't be obtained from the wall or from the Box, only when you first start or when you spawn back into the game at the beginning of the next round.