He completed vocational training in 1972 as a skilled worker for cattle breeding,[2] but was initially not allowed to study for political reasons.

He was therefore active as a swimming instructor, lifeguard and cattle breeder before he studied law at Karl Marx University Leipzig from 1974 to 1978.

As a high school graduate, Diestel said he joined the East German party CDU and left again after a few months.

On 1 July 1990, together with Wolfgang Schäuble, he signed the contract for the dismantling of the border fortifications between the GDR and the Federal Republic.

[2] In his term in 1990, there was a transfer of Stasi-intercepted files about West German politicians to the Federal Office for the Protection of the Constitution.

And that's why I didn't kick out colonels and generals […] During my term in office not a single politically motivated shot was fired […] there were no upheavals and I'm with these people just the way they were into German unity.

[10] Diestel sued journalists and publishers who had reported that he was "the last interior minister in the GDR to be jointly responsible for the destruction of some of the Stasi files".

On 10 July, 1995, the Hanseatic Higher Regional Court in Hamburg ruled that Diestel was politically responsible for the destruction of files from the Ministry for State Security during his tenure as Interior Minister of the GDR.

[11] Diestel later explained that he had campaigned politically for the files to be destroyed and had been supported in this regard by Chancellor Helmut Kohl and Interior Minister Wolfgang Schäuble.

In 2019, he said in retrospect, "The opinion later changed when it was noticed that the files that relate to the actual one's own comrade, namely from the old federal territory were already gone and that the Russians already have alike.

Before the free, democratic De Maizière government came to power, the state security had six to nine months to remove their own people from these files.

[2] Diestel has been running a law firm in Potsdam since 1993; he also manages other offices in Berlin, Leipzig, Güstrow and Zislow, his place of residence.

In 2004, he worked as legal counsel for PDS' leading candidate for the Landtag election in Saxony, Peter Porsch, who was suspected of being involved in Stasi activities.

According to Manfred Stolpe, Diestel is “an important and interesting personality in the transition from the GDR to German unity,"who courageously stood at the forefront "of the mass protests in Leipzig autumn 1989.

[23] Diestel was found guilty of Embezzlement in 2001, the Berlin district court imposed a fine of 9,900 marks on probation and issued a warning.

He bought the 3,500 square meter lake property from his ministry for 193,00 marks – estimated on the basis of the old GDR's price law.