[6] In 1941, Herbst was working as a tax consultant for an American oil company when he left civilian life and joined the Royal Canadian Air Force (RCAF) to learn to fly fighters in combat.

[6] After eight months, he transferred to Eglin Field on the Florida coast to work as a test pilot and to solve air combat tactical problems.

While Colonel David Lee "Tex" Hill was recuperating on the beach with his wife, he witnessed a daredevil pilot flying dangerous aerobatics very near the water.

"[6] Hill thought Herbst was an excellent candidate for air combat in the China Burma India Theater (CBI), and determined to get the flyer transferred.

[11] On his arrival in the CBI Theater, Herbst acquired the nickname "Pappy" because of his graying hair, his having a son in school, and his much greater age (34) relative to the other pilots.

[2] His ground crew painted a small swastika on his personal fighter aircraft in China, acknowledging his stated German kill though it was not verified.

[6] While flying a P-51B Mustang gathering weather data north of Kiatow on June 17, 1944, Herbst shot down a lone Nakajima Ki-43 fighter to make his first USAAF kill.

[13] The 74th transitioned to P-51C-7 Mustangs in August, and on September 3 Herbst shot down two Japanese aircraft that he reported as Aichi D3A "Val" dive bombers but were probably Mitsubishi Ki-51 "Sonia" models instead.

Using the radio call sign "Guerrilla", his squadron fought against Japanese forces conducting Operation Ichi-Go, a successful drive to consolidate communications along rail lines.

However, Herbst's idea of "administrative" flights was very broad and aggressive, including weather assessment and new pilot training in areas where Japanese aircraft were known to be operating.

On one such flight on September 5, 1944, Herbst was ferrying his Mustang fighter to a more sophisticated airfield in Liuliang, China, because the ejector chutes of three of its six machine guns were not working properly and needed expert attention.

Alone against the hostile force and with only half of his guns working, Herbst made a head-on attack against the top group, killing the leader.

With blood from deep scalp cuts partially blinding him, Herbst continued dogfighting, damaging other fighters and probably killing another flight leader.

During this time, Herbst teamed up with Major Carl T. Sigman to write an article for Popular Science entitled "How Planes Fight the 'White Devil' of the Air'", an explanation of the dangers of ice on aircraft, and how it is countered.

The two pilots performed a coordinated routine that thrilled the crowds at every stop, including an appearance at Washington, D.C.[8] Herbst named his P-80 "Jeanne" in honor of his bride-to-be.

[22][23] Flying in formation with Olds, Herbst crashed in a dry riverbed near the Del Mar Racetrack after his aircraft stalled during an encore of their routine finale in which the pair of P-80s did a loop while configured to land.