He was born in Stonarov, Moravia, then part of Austria-Hungary. He moved with his parents to Vienna in 1907 and later went to study law at the University of Vienna. At the start of World War I, he enlisted with the Austrian Army in August 1914 and was given a commission with the Tyrolean Kaiserjäger and served in Russia, Romania and also Italy. He was decorated for bravery on a number of occassions and while recovering from wounds in 1917 he completed his final examinations for his degree.
He went into law after the war and in 1921 set up his own practice. A successful lawyer, he was invited to join the cabinet of Chancellor Engelbert Dollfuss in 1933. Following the murder of Dollfuss he became a State Councillor from 1937 under Kurt von Schuschnigg. He joined the Austrian National Socialist party in 1931 and although he distanced himself from them when the party was made illegal in July 1934 he quickly renewed his membership in 1936, he was also close to the Vaterländische Front. In February 1938, at a meeting between Schuschnigg and Hitler at the Eagle's Nest (Hitlers residence in the Bavarian Alps), Hitler made several demands which Schuschnigg rejected. A plebiscite was planned for March but on March 11, under the threat of invasion, and with Austrian troops present, Schuschnigg resigned and Seyss-Inquart succeeded him at the head of a new National Socialist cabinet.
He liased between the Austrians and Germany for the Anschluss and he joined the German Nazi party in May 1938. He drafted the law reducing Austria to a province of Germany and remained head (Reichsstatthalter) of the newly named Ostmark, with Ernst Kaltenbrunner his chief minister and Burckel as Commissioner for the Reunion of Austria (concerned with the "Jewish Question"). Seyss-Inquart also received an honorary SS rank of "Gruppenführer" and in May 1939 he was made a Minister without portfolio.
Following the invasion of Poland, Seyss-Inquart became administrative chief for Southern Poland, but did not take up that post before the General Government was created, in which he became a deputy to the Governor General Hans Frank. It is claimed that he was involved in the movement of Polish Jews into ghettoes, in the seizure of strategic supplies and in the "extraordinary pacification" of the resistance movement.
Following the capitulation of the Low Countries he was appointed Reichskommissar for the Occupied Netherlands in May 1940, charged with directing the civil administration, with creating close economic collaboration with Germany and with defending the interests of the Reich. He supported the Dutch NSB and allowed them to create a paramilitary Landwacht, which acted as an auxiliary police force. Other political parties were banned in late 1941 and many former government officials were imprisoned at Sint-Michielsgestel. The adminstration of the country was largely controlled by Seyss-Inquart himself. He oversaw the politicisaton of cultural groups "right down to the chessplayers' club" through the Kulturkammer and set up a number of other politicised associations. Germany demanded occupation costs in the region of 50 million marks per month.
He introduced measures to combat 'terror' and when a widespread strike took place in Amsterdam, Arnhem and Hilversum in May 1943 special summary court-martial procedures were brought in and a collective fine of 18 million guilders was imposed. Up until the liberation Seyss-Inquart agreed to the execution of around 800 people, although some reports put this total at over 3,500, including the execution of people under the so-called "Hostage Law", the death of political prisoners who were close to being liberated, the Putten incident, and the reprisal execution of 230 Dutchmen for the attack on SS and Police Leader H. Rauter. From July 1944 the majority of Seyss-Inquart's powers were transferred to the military commander in the Netherlands and the Gestapo.
There were three major concentration camps in the Netherlands - Vught, Amersfoort and a "Jewish assembly camp" at Westerbork; there were a number of other camps variously controlled by the military, the police, the SS or Seyss-lnquart's administration. These included a "voluntary labour recruitment" camp at Ommen. In total around 530,000 Dutch civilians worked for the Germans, of whom 250,000 were sent to factories in Germany. There was an unsuccessful attempt by Seyss-Inquart to send all workers aged 21-23 to Germany, although he refused demands in 1944 for a further 250,000 Dutch workers and in that year sent only 12,000 people.
Seyss-Inquart was an open antisemite: on his arrival in the Netherlands he immediately took measures to remove Jews from the government, the media and leading positions in the economy. Anti-Jewish measures intensified from 1941, the 140,000 or so Jews were registered, ghettoes were created in Amsterdam and camps were set up at Westerbork and Vught, and in February 1941 1,000 Jews were sent to Buchenwald and Mauthausen concentration camp. Later the Dutch Jews were sent to Auschwitz. As Allied forces approached in September 1944 the remaining Jews at Westerbork were removed to Theresienstadt. Of the 140,000 registered only 13,500 survived the war.
When the Allies invaded the Netherlands the Nazi regime enacted a 'scorched earth' policy and destroyed docks and harbours to flood much of the country. The civilian population, with much agricultural land useless and with limited transport that could have moved food stocks for civilian use (partly due to civil disobedience), suffered in almost-famine conditions from September 1944 until early 1945, with around 30,000 Dutch people starving to death. Seyss-Inquart remained Reichskommissar until May 8, 1945, when, after a meeting with Karl Doenitz, he was captured in Hamburg.
At the Nuremberg Trials Seyss-Inquart faced charges of conspiracy to commit crimes against peace; planning, initiating and waging wars of aggression; war-crimes; and crimes against humanity. Defended by Gustav Steinbauer, he was found guilty of all charges except conspiracy. Sentenced to death by hanging, he was executed on October 16, 1946, together with the nine other Nuremberg defendants who had been sentenced to death.