He met politics first in 1833 when he attended to the assembly of Pozsony instead of his older brother, and this was the beginning of the carreer of one of the most important person in the Hungarian reformed politics of the 1840's. His name became first known due to his involvement in the suit of Miklós Wesselényi and the his success to declare the Hungarian Assembly's right to create laws.
In 1836 he wrote and distributed a document about the cases he supported without the permission of the censors; while it was confiscated it was already widespread and made his name familiar for the important circles. He was involved in creation of the 1839-40 laws of the Assembly and became honorary member of the Hungarian Scientific Academy.
After the death of his brother in 1842 he liberated his villeins (servants) and voluntarily chose to pay taxes to show that he is not just talking but really mean his reforms.
In 1846 after the bloody end of the Galitian-Polish uprising the reformers gained popularity and they released the "Ellenzéki nyilatkozat" (Manifesto of the Opposition) under the name of Deák, while it was really created by Kossuth.
While the revolution of 1848 he stayed calm and opposed violence as a political tool. He accepted position in the Batthyány government mostly to show his support of Batthyány.
After he was put on the rack he became the leader of the passive resistance. With the help of István Széchenyi his land was obtained again so he was able to lead the Hungarian politics from Budapest. He supported the "Conciliation" with all his possible strength.
After 1867 his health weakened, the continous work and the attacks used up most of his force. His reformist ideas were often rejected by the Parliament.
He died in January 28, 1876, and the Parliament created a law to remember his excellences and ordered that a statue should be created from national donations.