Direct proof of use
The MQ-9 Reaper is directly documented in the 2014 Yemen Civil War through U.S. and defense-reporting accounts of American aircraft operating over Yemen. U.S. Central Command said a U.S. MQ-9 was shot down over Yemen on June 6, 2019 by a Houthi SA-6 surface-to-air missile, and described the aircraft category in the statement title as U.S. observation aircraft.
Military Times summarized the same CENTCOM statement, reporting that an unmanned U.S. observation aircraft was shot down over Yemen on June 6 and identifying it as an MQ-9 Reaper. The supported public role for this conflict row is therefore surveillance and observation, not a documented MQ-9 strike mission in Yemen's civil-war battlespace.
Sources: CENTCOM Observation Aircraft Statement, Military Times June 2019 MQ-9 Report
Timeline
The clearest dated loss was June 6, 2019, when CENTCOM assessed that Houthi forces used an SA-6 surface-to-air missile to shoot down a U.S. MQ-9 over Yemen. CSIS Missile Threat placed the incident in a June 5-16 Yemen update alongside Houthi missile and drone attacks in the same period.
A second 2019 incident was reported on August 21, when Asharq Al-Awsat carried Reuters reporting that two U.S. officials said a U.S. military MQ-9 had been shot down late the previous day in Dhamar governorate, southeast of Houthi-controlled Sanaa. The same report said CENTCOM was investigating reports that a drone had been attacked in authorized airspace over Yemen.
Sources: CENTCOM Observation Aircraft Statement, CSIS Yemen Update June 2019, Asharq Al-Awsat Reuters August 2019 Drone Report
Operational role
The documented operator in the catalog record is the United States. Public sources for the Yemen civil-war row show U.S. MQ-9s present over Yemen and exposed to Houthi air-defense fire, but they do not establish a specific strike, target, or weapon release by an MQ-9 in this conflict.
The 2019 incidents also show the conflict's air-defense dimension. CENTCOM assessed the June shootdown as a Houthi SA-6 engagement enabled by Iranian assistance, while the August Reuters report attributed one official's preliminary assessment to a surface-to-air missile operated by Iran-aligned Houthi militias and noted another official's caution that responsibility was still being assessed.
Sources: CENTCOM Observation Aircraft Statement, Asharq Al-Awsat Reuters August 2019 Drone Report