The son of a Catholic master baker, Seidl grew up in Munich and studied law and economics from 1932 onward. He earned his doctorate in law in 1937 and completed the state bar examination in 1939. As a student and assistant to Edmund Mezger, a defense attorney who was loyal to the regime, he embraced the National Socialist interpretation of the law. He joined the SA in October 1934, the National Socialist Association of Legal Professionals (NSRB) in 1935, and the Nazi Party in 1937. From 1939 onward, Seidl worked in the office of the renowned Munich attorney Fritz Sauter. In 1940, he was called up for conscription, but did not have to go to the front and was allowed to continue his legal practice.
After the end of the war, Sauter and Seidl were appointed as defense counsel at the International Military Tribunal in Nuremberg. Seidl, the youngest attorney at the main Nuremberg trial, defended Hans Frank and Rudolf Hess; he advocated for the rehabilitation of Hess (died 1987) until his death. In the subsequent Nuremberg trials, he represented such people as Oswald Pohl, head of the SS Main Economic and Administrative Office. Seidl regained his license to practice law in German courts in 1948 after his denazification proceedings were discontinued. In the following decades, he became one of the most prominent attorneys for Nazi offenders in West German criminal trials.
He also pursued his political career within the Christian Social Union, Becoming a Bavarian Parliament member from 1958 to 1986, (deputy) parliamentary group leader from 1970 to 1974, state secretary in the Bavarian Ministry of Justice from 1974 to 1976 and Bavarian Minister of the Interior in 1977/78. Strictly anti-communist, Seidl advocated for the bolstering of domestic security and, in connection with the fight against the Red Army Faction (RAF), argued for the reintroduction of the death penalty. Seidl was awarded the Bavarian Order of Merit and the Great Cross of Merit of the Order of Merit of the Federal Republic of Germany. After his death it emerged that he had had links to the extreme right for decades, including to the publisher of the far-right National-Zeitung and to German People's Union (DVU) chairmanGerhard Frey, whom he legally advised along with Theodor Maunz . He was also in contact with the British Holocaust denier David Irving.
