Initially an educator and writer of textbooks, he was chairman of the Progressive Party for ten years, and Minister of Justice from 1927 to 1932.

Jónas was the author of the "immensely influential primer" Íslandssaga handa börnum ("History of Iceland for children", 2 vols., Reykjavík, 1915–1916).

[3] The textbook reflects the nationalist account of Icelandic history (originally introduced by Jón Jónsson Aðils) wherein the Icelandic nation prospered as an independent nation during the medieval period but suffered degradation for centuries after it gave up its sovereignty to the Norwegian King by signing the Old Covenant (1262).

[1] According to historian Thorsteinn Helgason, Jónas' textbook presents the Icelandic nation "as a homogenous and united flock, while foreigners, mainly the Danes, are depicted as evil.

More or less fictional figures from mediaeval sagas, together with literary and political personalities of all eras, populate the scene.