Direct proof of use
The MiG-21 is directly documented in the 1990 Gulf War through U.S. Air Force historical material on Iraqi aircraft losses. Daniel L. Haulman's Air Force Historical Research Agency paper says U.S. Air Force pilots shot down 37 Iraqi aircraft during Operation Desert Storm between January and March 1991, including two MiG-21s.
A Robins Air Force Base photo release gives the dated event for those MiG-21 losses: on February 6, 1991, Capt. Thomas Dietz shot down two Iraqi MiG-21s over Iraq with AIM-9 missiles while flying an F-15C assigned to the 36th Tactical Fighter Wing. Air & Space Forces Magazine separately identifies the same engagement as a combat air patrol in which Dietz downed the pair of MiG-21s and Lt. Robert Hehemann downed two Su-25s.
Sources: What Happened to the Iraqi Air Force?, Robins F-15 Photo Release, The Missing Aces
Timeline
Operation Desert Storm's air campaign opened in January 1991 after the 1990 Iraqi invasion of Kuwait and the coalition buildup. The direct MiG-21 evidence in this record comes from the air-superiority phase of that campaign, not from transfer or possession records: Iraq was the operator, and the documented event is combat employment resulting in two aircraft losses.
On February 6, 1991, the documented MiG-21 encounter occurred over Iraq. Robins Air Force Base attributes both MiG-21 shootdowns to Capt. Thomas Dietz in F-15C serial 79-078, while Air & Space Forces Magazine places the event on combat air patrol and describes the aircraft as part of a four-aircraft Iraqi group that also included two Su-25s.
Sources: What Happened to the Iraqi Air Force?, Robins F-15 Photo Release, The Missing Aces
Narrative
In this conflict, the MiG-21 appears as part of Iraq's fighter force opposing the U.S.-led coalition air campaign. The catalog role is air defense because the documented use is Iraqi fighter employment against coalition air superiority operations rather than a confirmed strike mission by the type.
The available direct sources do not describe MiG-21 sorties in detail beyond the February 6 engagement and aggregate Iraqi aircraft-loss accounting. They do, however, establish the key conflict-use claim: Iraqi MiG-21s were fielded during Operation Desert Storm, and two were destroyed in air-to-air combat by a coalition F-15C.
Sources: What Happened to the Iraqi Air Force?, Robins F-15 Photo Release, The Missing Aces