After his father's death in 1824, he moved to Ljubljana, where he was raised by his uncle Mihael Dežman [sl], who was a financial supporter of the Slovene national revival, and a personal friend of the philologist Franc Metelko.
After finishing high school in Ljubljana and Salzburg in 1839, he enrolled in the University of Vienna, where he studied medicine and law.
Among others, he participated in the public funeral of the Polish exile patriot Emil Korytko in Ljubljana, and was chosen to carry his coffin.
During the Revolution of 1848, he supported the United Slovenia program, and helped organize the boycott of the elections to the Frankfurt Parliament in the Slovene Lands.
Among other things, he wrote a bibliography of the poet Valentin Vodnik, and compiled the natural science terminology for Maks Pleteršnik's Slovene–German dictionary.
Dežman began his political career in the Slovene National Movement, but in the mid-1850s, he became alienated from it, disenchanted with the conservatism and pragmatism of its leaders Janez Bleiweis and of Lovro Toman.
In 1875, he started archeological excavations on the Ljubljana Marsh, which brought to the discovery of prehistorical pile dwellings at Ig.