Rašín was born into a family of farmers. After gymnasium he continued his schooling with the study of law at the Charles University in Prague. Here he became active in politics, and a leader in the radical students movement. In the Omladina Trial (an 1894 trial against radical Czech youths), he was sentenced to two years in prison. Rašín continued his political activity although he became less radical over time. In 1911, he was elected into the parliament of the Austrian monarchy. After the start of World War I Rašín supported the Czech separatist movement. In 1915, he was imprisoned and sentenced to death (together with Karel Kramář). With the death of emperor Franz Jozeph I his sentence was commuted and in 1917 Rašín received amnesty. He immediately returned to politics.
In 1918, Rašín and others organised the establishment of Czechoslovakia as independent state. From 1918-19, he was the first Finance Minister of the new state. At this position he managed to stabilize the currency and avoid inflation that was damaging neighbouring countries. His authoritative and uncompromising behaviour helped him to achieve his aims but generated lot of animosity.
In 1922, Rašín again became Minister of Finance in the government of Antonín Švehla. Amidst an economic crisis he stressed the politics of deflation (in 1922 prices dropped by 42%, salaries by 32%) and a strong currency. High unemployment caused great animosity towards him, especially from the left. A fierce anti-Rašín campaign developed. On January 5, 1923, Rašín was shot in Prague by young anarchist Josef Šoupal (1903-59). Rašín died after a long period of suffering on February 18, 1923.