Direct proof of use
ATACMS is directly documented in the 1990 Gulf War through U.S. Army and Department of Defense accounts of Operation Desert Storm. The Defense Department's final report to Congress states that ATACMS was one of two MLRS munitions used during Operations Desert Shield and Desert Storm, and that two deep-attack-capable MLRS batteries configured to fire ATACMS deployed to Saudi Arabia.
The same report describes ATACMS fired from the M270 MLRS launcher and used against surface-to-air missile sites, rocket and artillery batteries, logistics and refueling sites, and convoys. It also records a first-night joint suppression-of-enemy-air-defenses mission in which 10 ATACMS missiles attacked an Iraqi air-defense site.
Sources: DoD Conduct of the Persian Gulf War, U.S. Army Then & Now ATACMS to PrSM
Timeline
ATACMS reached the theater as an accelerated early-production system. The Defense Department report says ATACMS became first operational in August 1990, production was accelerated after the crisis began, two ATACMS-capable MLRS batteries deployed to Saudi Arabia, and a conversion team deployed with deep battle kits to make standard MLRS batteries ATACMS-capable.
During the opening air campaign, ATACMS contributed to suppression of Iraqi air defenses. The Defense Department report places 10 ATACMS missiles in the first-hour joint SEAD effort, while later Army acquisition writing identifies the 6th Battalion, 27th Field Artillery Regiment as the first unit equipped with ATACMS and the first to use it in combat during Operation Desert Storm.
Sources: DoD Conduct of the Persian Gulf War, USAASC Preparation for Operational Testing
Operational role
In Desert Storm, ATACMS gave U.S. ground commanders a corps-level deep-strike missile fired from modified M270 MLRS launchers. The Defense Department described the system as the operational commanders' deep-strike weapon for soft, stationary, and semi-fixed targets such as surface-to-surface missile sites, air-defense sites, logistics sites, and command, control, communications, and intelligence facilities.
The publicly documented mission set was not limited to deployment or possession. The Defense Department reported that ATACMS attacks apparently silenced targeted air-defense sites, that coalition aircraft using corridors cleared by ATACMS strikes reported no enemy air-defense radar activity, and that one short-notice ATACMS strike requested by A-10 pilots destroyed an air-defense target within minutes.
Open sources differ in detail. The official Defense Department report gives target categories and several performance examples but does not publish a complete missile-by-missile firing log. GlobalSecurity, summarizing Army TACMS background, reports 105 missiles deployed to Southwest Asia and 32 fired, including the first two combat ATACMS launches on January 18, 1991.
Sources: DoD Conduct of the Persian Gulf War, GlobalSecurity M39 Army TACMS, Redstone Army TACMS, CSIS ATACMS Background