He then studied at the Liepoja Gymnasium from 1920 he served in the electrical engineering battalion of the Lithuanian army.
in 1923 Assistant Head of the Organization Department of the Post, Telegraph and Telephone Board.
[1] In 1924, he graduated from the Adult Gymnasium of the Lithuanian Teachers' Trade Union in Kaunas and entered the Technical Faculty of the University of Lithuania.
In 1925, he was transferred to the Physics and Chemistry Department of the Faculty of Mathematics and Natural Sciences.
From 1925, Mautlis participated in the activities of the Lithuanian Social Democratic Party and was a member of its student organization Žiežirba.