Now increasingly living as a male, during his doctorate studies Prenner joined the
Osvobodilna fronta (Liberation Front, OF), an anti-fascist civil resistance movement. His apartment was used for meetings and as a dropbox for partisan communiques. He housed comrades who needed shelter for a night and provided legal advice or defense to detainees and prisoners. As he refused to join the
communist party on account of their methods rather than their ideology, party leaders began to watch him as a potential opponent. Prenner opened a private law practice in 1943, and gained a reputation for his work during the war with political prisoners. In one scheme, he filed false paperwork with the authorities claiming that the judgments of the Italian courts were invalid after Italy
capitulated to the Allies. He secured the freedom of many Slovenes before the Germans realized their error and arrested him in 1944. He escaped imprisonment, but had to pay a large fine. When the war ended, Prenner was one of only 13 lawyers allowed to continue practicing by the communist government. In 1946, he was assigned to represent the partisan Tončko Vidic, who had been sentenced to death for treason. Prenner thought that the conviction was based on flimsy evidence and found witnesses who were able to prove that the charges were false. He demanded a retrial, earning an acquittal for his client. In the aftermath of the trial, he and the prosecutor, whose weak case he had exposed, engaged in a physical altercation, which resulted in Prenner being fined by the disciplinary court. He became known as a lawyer who rarely lost a case, using unyielding determination and
legal acumen. Among those he represented who were accused of collaborating with the Italian occupation authorities were
Juro Adlešič, former mayor of Ljubljana; the owner of the cinema in Ljubljana; and Slovenes who had participated with
Draža Mihailović in resistance during the war. Prenner represented those charged with crimes against the state or state property, refusing to represent clients he did not consider to be wrongly accused. He believed that the
judiciary had to maintain its independence from the government and not be swayed by politics. This often led to confrontations with those who saw the courts as an enforcer for the communist regime. In 1947, the communist newspaper,
Slovenski poročevalec (The Slovene Reporter), published an article strongly criticizing Prenner. It alleged that he undermined the reputation and authority of prosecutors, the court system, the
State Security Administration, and the government by inappropriately belittling the performance of officials. He was arrested and spent a month and a half in a pre-trial jail located on
Miklošičeva cesta (Miklosich Street). While in jail, he composed the libretto
Slovo od mladosti (Farewell from Youth). Though Prenner's comedy
Veliki mož (The Great Man) was successfully staged in 1943 at the
Ljubljana Drama Theatre, in 1947 it was attacked by the literary censors and he was expelled from the Writers Association. He was not allowed to publish again until 1976. Released from prison in the fall of 1947, he began working part-time at the Institute of the Slovene Language, part of the
Slovenian Academy of Sciences and Arts, advising on legal concepts for the Slovene language dictionary. He was arrested by the State Security Administration in 1949 and returned to the jail on
Miklošičeva cesta. Officially, he was never charged with an offense and kept his job until 1952. He was moved from the pre-trial jail to the
Ferdreng Women's Camp and required to work in the quarry. To keep up his spirits and those of the other inmates, Prenner wrote the humorous piece,
Prošnja za novo stranišče (Request for a New Toilet), stressing the inadequacy of three toilets for 300 prisoners. He helped the other prisoners by filing complaints about the poor food and sexual assaults, which often earned him time in solitary confinement. After a new complaint was made against Prenner later in 1949, he was transferred to the Ljubljana District Court Prison, but when the charges were withdrawn, he was sent to the women's prison at
Brestanica castle. In January 1950, he was again transferred and imprisoned in the castle at
Škofja Loka, where he wrote the play,
Vasovalci. He was released without receiving either an indictment or a pardon in May 1950, but had no job to return to, as his law license had been revoked. Initially he lived in
Rožna Dolina, with his long-term female partner, Slavica Jelenc, who was a mathematics professor. When his brother-in-law Josip Šerbec died, Prenner moved to
Bežigrad, where his sister lived. Unable to sell his writing, he took care of the family, helped his nephews who were law students, and worked as a clerk in a law office. Unable to earn enough money to support the family, he and his sister sold their parents' home in Slovenj Gradec, which they had inherited after their parents died in 1945. ==Later life (1954–1976)==