Born in Veľká Bytča (today's Bytča), he graduated from the "Pasmaneum" in Vienna in 1910 as a theologian, and afterwards was a Catholic curate in several towns, teaching Slovak spelling, organising theatre performances and doing cultural work. At the beginning of World War I he was a military chaplain. In 1915 he became the director of the Theological Seminary of Nitra and teacher at the Piarist High School in the same town. From 1921 to 1924 - i.e. at a very early age - he was the secretary of the bishop and teacher at the Seminary of Divinity at Nitra. In 1924 he became the dean and parish priest of the town of Bánovce nad Bebravou.
Tiso was also one of the leaders of the Slovak Popular Party, a Roman Catholic and, during World War II a Fascist leaning party, that sought the autonomy of Slovakia within Czechoslovakia and was the biggest party in Slovakia since 1923. It was one of the two purely Slovak parties in Slovakia; the remaining parties were either parties of national minorities, or at were least nominally active in all Czechoslovakia. This party was founded by Father Andrej Hlinka in 1913 while Austria-Hungary still ruled Slovakia. When Hlinka died in 1938, Tiso de-facto became its leader (officially he was vice-leader of the party from 1930 to October 1, 1939, and only party leader after that date). Even during his presidency, Tiso continued to work actively as the parish priest of the town of Bánovce nad Bebravou (from 1924 to 1945). From 1925 to 1939, he was a deputy to the Czechoslovak parliament in Prague, and from 1927 to 1929 a member of the Federal government - the Minister of Health and Sports.
After Adolf Hitler invaded Sudetenland in the Czech part of Czechoslovakia and the Czechoslovak president Eduard Beneš fled the country in late 1938, during the chaos which arose in Bohemia and Moravia and Silesia, the Slovaks (not having any form of autonomy within Czechoslovakia until then) declared their autonomy within Czechoslovakia and Tiso, as the leader of one of the Slovak parties - the "Hlinka's Slovak Popular Party"- , became the prime minister of this autonomous Slovakia (only until March 9, 1939). Hungary, having never really accepted the separation of Slovakia from Hungary in 1918, took advantage of the situation and managed to persuade Germany and Italy to force Slovakia to let Hungary occupy one third of Slovak territory in November 1938 (by the so-called Vienna Arbitration).
Facing this new menace, all Czech or Slovak political parties in Slovakia (except for the Communist party) voluntarily joined their forces and created the "Hlinka's Slovak Popular Party - Party of Slovak National Unity" in November 1938, which created the basis for the future authoritarian regime in Slovakia. (The same happened in the Czech part of the country two weeks later for Czech parties.) In January 1939, the Slovak government officially prohibited other parties than the Party of Slovak National Unity, the "Deutsche Partei" (a party of Germans in Slovakia) and the "Unified Hungarian Party" (a party of Hungarians in Slovakia).
Since February 1939, representatives of Germany - planning to occupy Bohemia and Moravia and Silesia and basically not interested in Slovakia - started to officially persuade Slovak politicians to declare the independence of Slovakia. On March 9, 1939, Czech troops occupied Slovakia and Tiso was deposed as prime minister. On March 13, 1939, Adolf Hitler lost his patience and Tiso - as the deposed prime minister - was invited to Berlin, and was forced by Hitler in person to immediately (as he said "in a flash") declare the independence of Slovakia under German "protection", otherwise Germany would allow Hungary (and partly Poland) to annex the remaining territory of Slovakia. Under these circumstances, Tiso spoke by phone to the Czechoslovak president Emil Hácha and to the then prime minister of Slovakia Karol Sidor and they agreed to convene the Slovak parliament the next day and let it decide. On March 14, the Slovak parliament unanimously declared independence of Slovakia, and on March 15, Germany invaded (remaining) Bohemia and Moravia - exactly according to German plans.
Tiso was the prime minister of independent Slovakia from March 14 until October 26, 1939, and since October 26 the president of Slovakia (prime ministers being other persons). On October 1, he was officially appointed the president of the Slovak Popular Party.
The "independence" of Slovakia was largely illusory in the sense that Slovakia was a German puppet state. The same applies to most of the countries of Central Europe. On the other hand, Slovakia was independent of Prague and not more dependent on Nazi Germany than post-WWII Czechoslovakia on the Soviet Union (German troops fully occupied Slovakia only at the end of the war in 1944, while Soviet troops were in Czechoslovakia for more than 40 years).
The Slovak Popular Party was almost the sole legal political organisation in Slovakia. Tiso submitted to Nazi demands for anti-Semitic legislation in Slovakia. Tiso himself - as a Catholic priest - was silently supporting the deportations of Jews. He granted only 2,000 (the exact number is disputed) presidential exceptions from the anti-semitic Jewish Code to wealthy Jews who agreed to baptism. The deporations of Jews from Slovakia started in March 1942, but were stopped - despite heavy opposition from Germany - in October 1942, when it became clear that the Jews are not "only" misused as workers but are killed in German camps, and when public protests arose. Slovakia was the first state of the Nazi sphere to stop deportations of Jews. Between October 1942 and October 1944, the country even served as a last resort for Jews that were persecuted in neighbouring countries. The deportations resumed in October 1944, when the Soviet army reached the Slovak border, and the Slovak National Uprising took place and, as a result, Germany decided to occupy entire Slovakia and the country lost its independence.
See also: History of Slovakia