On 7 March 1945, Fieldorf was arrested by the Soviet NKVD in the town of Milanówek.
Initially, he was misidentified under the name Walenty Gdanicki and sent to a forced labour camp in the Ural Mountains.
Released in 1947, he returned to new Poland ruled by the communist Polish Workers' Party government and the increasingly repressive Ministry of Public Security.
He settled in Biała Podlaska under his assumed name and did not return to underground activities.
Moving between Warsaw and Kraków, he eventually settled in Łódź.

The government, which was persecuting former resistance members loyal to the London-based government-in-exile, offered an amnesty to them, in 1948.
Not knowing that the amnesty was a sham, Fieldorf outed himself to the authorities.
He was then placed under investigatory arrest in Warsaw.
In prison he refused to collaborate with the Communist security services, even under torture.
General Fieldorf's brutal interrogations were personally supervised by MBP colonel Józef Różański.
Kazimierz Gorski, Polish secret police, the UB interrogator, testified in 1997: "[Józef] Różański would stop by frequently during many of my interrogations of general [August] Fieldorf, and he would have conversations with him on many subjects.
The prosecuting attorney Benjamin Wajsblech would show up frequently as well, and would, on many occasions, give me verbal instructions.
I prepared a decision to refuse the general's [defense] evidence materials.
I wrote it under the dictation of Wajsblech.
I didn't decide as to whom, and how, I should interrogate".

Fieldorf was accused by prosecutor Helena Wolińska-Brus of being a "fascist-Hitlerite criminal" and having ordered an execution of Soviet partisans while serving in the AK.
After a kangaroo court trial, he was sentenced to death on 16 April 1952 by the presiding judge Maria Gurowska.
An appeal to a higher court failed, and the family's plea for a pardon was denied by then the communist leader Bolesław Bierut who refused to grant clemency.
The sentence was carried out, by hanging, on 24 February 1953 at 3:00 pm in the infamous Mokotów Prison in Warsaw.

The Communist Prosecuting Attorney Wiktor Gattner described General Fieldorf's last moments as follows:
I asked the condemned if he had any wishes.
Fieldorf responded: 'Please notify my family'.
I stated that his family would be notified [...] The condemned persistently looked straight into my eyes.
He stood erect.
No one was holding him.
He made an appearance of a very strong man.
One would almost admire his composure amidst such dramatic events.
He neither screamed, nor made any gestures.
I said: Carry out [the execution]!
The executioner and one of the guards approached the condemned […] I went to see the warden afterwards, and then by my own hand I prepared the protocol of the execution.

General Fieldorf's body was never returned to his family, and remains buried in an unknown location to this day.
In 2009, an article in a British newspaper suggested that Fieldorf was buried in a mass grave in a Warsaw cemetery, together with the remains of 248 other murdered Polish non-communists.

In 1958 the prosecutor's office discontinued any further investigations.
On 7 March 1945, Fieldorf was arrested by the Soviet NKVD in the town of Milanówek. Initially, he was misidentified under the name Walenty Gdanicki and sent to a forced labour camp in the Ural Mountains. Released in 1947, he returned to new Poland ruled by the communist Polish Workers' Party government and the increasingly repressive Ministry of Public Security. He settled in Biała Podlaska under his assumed name and did not return to underground activities. Moving between Warsaw and Kraków, he eventually settled in Łódź.
The government, which was persecuting former resistance members loyal to the London-based government-in-exile, offered an amnesty to them, in 1948. Not knowing that the amnesty was a sham, Fieldorf outed himself to the authorities. He was then placed under investigatory arrest in Warsaw. In prison he refused to collaborate with the Communist security services, even under torture. General Fieldorf's brutal interrogations were personally supervised by MBP colonel Józef Różański. Kazimierz Gorski, Polish secret police, the UB interrogator, testified in 1997: "[Józef] Różański would stop by frequently during many of my interrogations of general [August] Fieldorf, and he would have conversations with him on many subjects. The prosecuting attorney Benjamin Wajsblech would show up frequently as well, and would, on many occasions, give me verbal instructions. I prepared a decision to refuse the general's [defense] evidence materials. I wrote it under the dictation of Wajsblech. I didn't decide as to whom, and how, I should interrogate".
Fieldorf was accused by prosecutor Helena Wolińska-Brus of being a "fascist-Hitlerite criminal" and having ordered an execution of Soviet partisans while serving in the AK. After a kangaroo court trial, he was sentenced to death on 16 April 1952 by the presiding judge Maria Gurowska. An appeal to a higher court failed, and the family's plea for a pardon was denied by then the communist leader Bolesław Bierut who refused to grant clemency. The sentence was carried out, by hanging, on 24 February 1953 at 3:00 pm in the infamous Mokotów Prison in Warsaw.
The Communist Prosecuting Attorney Wiktor Gattner described General Fieldorf's last moments as follows:
I asked the condemned if he had any wishes. Fieldorf responded: 'Please notify my family'. I stated that his family would be notified [...] The condemned persistently looked straight into my eyes. He stood erect. No one was holding him. He made an appearance of a very strong man. One would almost admire his composure amidst such dramatic events. He neither screamed, nor made any gestures. I said: Carry out [the execution]! The executioner and one of the guards approached the condemned […] I went to see the warden afterwards, and then by my own hand I prepared the protocol of the execution.
General Fieldorf's body was never returned to his family, and remains buried in an unknown location to this day. In 2009, an article in a British newspaper suggested that Fieldorf was buried in a mass grave in a Warsaw cemetery, together with the remains of 248 other murdered Polish non-communists. 
In 1958 the prosecutor's office discontinued any further investigations.
Emil August Fieldorf