The first G.91s entered service with the Italian Air Force in August 1958, with 103mo Gruppo, 5a Aerobrigata, called "Caccia Tattici Leggeri (CTL)", based at Pratica di Mare Air Force Base, the same with Reparto Sperimentale di Volo. The next operational unit was 14mo Gruppo, Seconda Aerobrigata in 1961. This unit had its role shifted to tactical support, because its groups were 14mo, 103mo (dispatched from 5 A/B to this Aerobrigade) and 13mo (only in reserve). All of them were based at Treviso-Sant'Angelo.
Forty-five G.91 T/3 Fiat built two-seat trainer aircraft were ordered for the Luftwaffe, the first 35 being allocated to Waffenschule 50 (Weapon School 50) with the balance of the order divided between operational units. 22 aircraft were built by Dornier between 1971 and 1973, these aircraft were used to train Weapons Systems Officers for the F-4 Phantom.
Five Fiat built G.91R/3 aircraft were delivered to Erprobungstelle 61 for trials with subsequent deliveries being allocated to Aufklarungsgeschwader 53 based at Erding, near Munich along with Waffenschule 50. The first Dornier-built example of this variant was flight tested on 20 July 1961. The G.91R/3 equipped four newly formed Leichte Kampfgeschwader (light attack wings), often replacing former F-84 Thunderstreak units.
Fifty G.91 R/4 aircraft were taken up from a cancelled Greek/Turkish order but being unsuitable for operational use were used as training aircraft and were operated solely by Waffenschule 50. When the initial training programme was completed the R4 was retired in 1966, 40 surviving airframes were sold to Portugal. Other R/4 aircraft remained in Germany and were transferred to ground instructional use or displayed at recruitment presentations.
On 1 January 1970, the Luftwaffe fleet consisted of 310 G.91 R/3 and 40 G.91T aircraft, and by 1976 only 20 of the R/3s had been lost to accidents, a loss rate of 6%. The G.91 R/3 was to be replaced in the early 1980s by the Dassault/Dornier Alpha Jet which operated in the same role, with the last G.91s being retired in 1982.
From 1961, Portugal became involved in fighting against nationalist movements in its African overseas territories, the series of conflicts becoming known as the Portuguese Colonial War. Portugal had deployed a detachment of F-86 Sabres to Portuguese Guinea in August 1961, prior to the outbreak of major fighting, but was forced to withdraw the jet fighters back to Europe owing to pressure from the United States and the United Nations, who imposed an arms embargo. This left a gap in air cover for Portugal's African colonies, both in the close air support role, and in the air defence role.
In 1965, as the scale of fighting increased, Portugal attempted to purchase 100 surplus Canadian built Sabre Mk 6s from West Germany, but instead, it was offered 40 G.91R/4s, which had originally been built for Greece and Turkey and which differed from the rest of the Luftwaffe G.91s sufficiently to create maintenance problems in exchange for allowing Germany to build and use an airbase at Beja in Portugal for training.
G.91s arrived in Portuguese Guinea in 1966, equipping Esquadra 121 Tigres based at Bissau, and being used for reconnaissance and close support with rockets, napalm and bombs against PAIGC rebels. When the PAIGC started to be supplied with Soviet-made Strela 2 (NATO designation SA-7 Grail) MANPADS in early 1973, these immediately became a threat to Portuguese air superiority. On 25 March 1973, and 28 March, two FAP G.91s were shot down by missiles within three days, with a further two lost to conventional ground fire later in the year. (By comparison, only two G.91s had been lost in Guinea from 1966 to 1973.) A final G.91 was lost to a missile on 31 January 1974, while Strelas were also responsible for the loss of a T-6 Texan and two Do.27K-2s.
G.91s deployed to Mozambique at the end of 1968, equipping Esquadra 502 Jaguares ininitally at Beira, later moving to Nacala, with a second squadron Esquadra 702 Escorpiões (Scorpions) forming in September 1970 at Tete, flying against FRELIMO forces. Fremilo also received Strelas in 1973, although unlike elsewhere, the Portuguese in Mozambique did not lose any aircraft to missiles with Chinese support, even if it forced Portuguese pilots to change their tactics. The only G.91 destroyed in combat in Mozambique was the serial number 5429, flown by Lt. Emilio Lourenço: his plane was destroyed and Lourenço killed by a premature detonation of its bombs while flying a strike against rebel positions on 15 March 1973.
In 1973, with the United Nations weapons embargo against Portugal, the Air Force faced problems purchasing further numbers of close air support aircraft. An attempt was then made to acquire more Fiat G.91s from Germany by having Dornier disassembling the aircraft and then selling them as spare parts to Switzerland and Spain. These spare parts would be later sold to Portugal and assembled locally with different serial numbers. However, the deal did not follow through as the German government vetoed it.
In April 1974, the Portuguese government fell in the Carnation Revolution, with the new government seeking to grant its colonies independence. Portugal withdrew its G.91s from Guinea when it was granted independence in 1974, with its forces also leaving Mozambique. One of the G.91 squadrons was briefly deployed to Angola in late 1974, in order to try to prevent fighting between rival National Liberation Front of Angola (FNLA) and National Union for the Total Independence of Angola (UNITA) forces, being finally withdrawn back to Portugal in January 1975.
In 1976, a second purchase of 14 G.91 R/3s and 7 G.91 T/3 trainers was made from Germany, which were followed by further aircraft when the G.91 was withdrawn from Luftwaffe service in from 1980 to 1982, giving a total of 70 R/3s and 26 T/3s, although not all of these entered service, with many being broken up for spare parts. Portugal finally phased out the last of its G.91s in 1993.