The first contract for the production F-5A was issued in 1962, the first overseas order coming from the Royal Norwegian Air Force on 28 February 1964.
It entered service with the 4441st Combat Crew Training Squadron, USAF, at Williams Air Force Base, which had the role of training pilots and ground crew for customer nations, on 30 April that year.
At that point, it was still not intended that the aircraft be used in significant numbers by the USAF itself.

This changed with testing and limited deployment in 1965.
Preliminary combat evaluation of the F-5A began at the Air Proving Ground Center, Eglin AFB, Florida, during the summer of 1965 under project Sparrow Hawk, with one airframe lost through pilot error on 24 June.
In October 1965, the USAF began a five-month combat evaluation of the F-5A titled Skoshi Tiger.
A total of 12 aircraft were delivered for trials to the 4503rd Tactical Fighter Squadron, and after modification with probe and drogue aerial refueling equipment, armor and improved instruments, were redesignated F-5C. Over the next six months, they performed combat duty in Vietnam, flying more than 2,600 sorties, both from the 3rd Tactical Fighter Wing at Bien Hoa over South Vietnam and from Da Nang Air Base where operations were flown over Laos.
Nine aircraft were lost in Vietnam, seven to enemy ground fire and two to operational causes.
Although declared a success, with the aircraft generally rated as capable a ground-attack aircraft as the F-100, but suffering from a shorter range, the program was considered a political gesture intended to aid the export of more F-5s than a serious consideration of the type for U.S. service.
From April 1966, the aircraft continued operations as 10th Fighter Commando Squadron with their number boosted to 17 aircraft. (
Following Skoshi Tiger the Philippine Air Force acquired 23 F-5A and B models in 1965.
These aircraft, along with remanufactured Vought F-8 Crusaders, eventually replaced the Philippine Air Force's F-86 Sabre in the air defense and ground attack roles.)

In June 1967, the 10th FCS's surviving aircraft were supplied to the Republic of Vietnam Air Force (RVNAF), which previously had only Douglas A-1 Skyraider attack aircraft.
This new RVNAF squadron was titled the 522nd Fighter Squadron.
The president of South Vietnam had originally asked for F-4 Phantoms used by the Americans, but the RVNAF flew primarily ground support as the Communist forces employed no opposing aircraft over South Vietnam.
Following the Fall of Saigon in 1975, captured examples were used operationally by the Vietnam People's Air Force (VPAF), in particular against Khmer Rouge.
In view of the performance, agility and size of the F-5, it might have appeared to be a good match against the similar MiG-21 in air combat; however, U.S. doctrine was to use heavy, faster and longer-range aircraft like the Republic F-105 Thunderchief and McDonnell Douglas F-4 Phantom II over North Vietnam.
41 F-5s were captured by the People's Army of Vietnam when they defeated South Vietnam on 30 April 1975; of the captured equipment, the Soviets took delivery of a complete F-5E, along with various spare parts and support equipment; aircraft would arrive in Poland and Russia for study of U.S. aviation technology, while others were decommissioned and put on display at museums in Vietnam.

The F-5 was also adopted as an opposing forces (OPFOR) "aggressor" for dissimilar training role because of its small size and performance similarities to the Soviet MiG-21.
In realistic trials at Nellis AFB in 1977, the F-14 reportedly scored slightly better than a 2:1 kill ratio against the simpler F-5, while the F-15 scored slightly less.
There is some contradiction of these reports, another source reports that "For the first three weeks of the test, the F-14s and F-15s were hopelessly outclassed and demoralized"; after adapting to qualities of the F-5 and implementing rule changes to artificially favor long range radar-guided missiles, "the F-14s did slightly better than breaking even with the F-5s in non-1 v 1 engagements; the F-15s
The first contract for the production F-5A was issued in 1962, the first overseas order coming from the Royal Norwegian Air Force on 28 February 1964. It entered service with the 4441st Combat Crew Training Squadron, USAF, at Williams Air Force Base, which had the role of training pilots and ground crew for customer nations, on 30 April that year. At that point, it was still not intended that the aircraft be used in significant numbers by the USAF itself.
This changed with testing and limited deployment in 1965. Preliminary combat evaluation of the F-5A began at the Air Proving Ground Center, Eglin AFB, Florida, during the summer of 1965 under project Sparrow Hawk, with one airframe lost through pilot error on 24 June. In October 1965, the USAF began a five-month combat evaluation of the F-5A titled Skoshi Tiger. A total of 12 aircraft were delivered for trials to the 4503rd Tactical Fighter Squadron, and after modification with probe and drogue aerial refueling equipment, armor and improved instruments, were redesignated F-5C. Over the next six months, they performed combat duty in Vietnam, flying more than 2,600 sorties, both from the 3rd Tactical Fighter Wing at Bien Hoa over South Vietnam and from Da Nang Air Base where operations were flown over Laos. Nine aircraft were lost in Vietnam, seven to enemy ground fire and two to operational causes. Although declared a success, with the aircraft generally rated as capable a ground-attack aircraft as the F-100, but suffering from a shorter range, the program was considered a political gesture intended to aid the export of more F-5s than a serious consideration of the type for U.S. service. From April 1966, the aircraft continued operations as 10th Fighter Commando Squadron with their number boosted to 17 aircraft. (Following Skoshi Tiger the Philippine Air Force acquired 23 F-5A and B models in 1965. These aircraft, along with remanufactured Vought F-8 Crusaders, eventually replaced the Philippine Air Force's F-86 Sabre in the air defense and ground attack roles.)
In June 1967, the 10th FCS's surviving aircraft were supplied to the Republic of Vietnam Air Force (RVNAF), which previously had only Douglas A-1 Skyraider attack aircraft. This new RVNAF squadron was titled the 522nd Fighter Squadron. The president of South Vietnam had originally asked for F-4 Phantoms used by the Americans, but the RVNAF flew primarily ground support as the Communist forces employed no opposing aircraft over South Vietnam. Following the Fall of Saigon in 1975, captured examples were used operationally by the Vietnam People's Air Force (VPAF), in particular against Khmer Rouge. In view of the performance, agility and size of the F-5, it might have appeared to be a good match against the similar MiG-21 in air combat; however, U.S. doctrine was to use heavy, faster and longer-range aircraft like the Republic F-105 Thunderchief and McDonnell Douglas F-4 Phantom II over North Vietnam. 41 F-5s were captured by the People's Army of Vietnam when they defeated South Vietnam on 30 April 1975; of the captured equipment, the Soviets took delivery of a complete F-5E, along with various spare parts and support equipment; aircraft would arrive in Poland and Russia for study of U.S. aviation technology, while others were decommissioned and put on display at museums in Vietnam.
The F-5 was also adopted as an opposing forces (OPFOR) "aggressor" for dissimilar training role because of its small size and performance similarities to the Soviet MiG-21. In realistic trials at Nellis AFB in 1977, the F-14 reportedly scored slightly better than a 2:1 kill ratio against the simpler F-5, while the F-15 scored slightly less. There is some contradiction of these reports, another source reports that "For the first three weeks of the test, the F-14s and F-15s were hopelessly outclassed and demoralized"; after adapting to qualities of the F-5 and implementing rule changes to artificially favor long range radar-guided missiles, "the F-14s did slightly better than breaking even with the F-5s in non-1 v 1 engagements; the F-15s 
Northrop F-5