He was ordained on 21 July 1855[2] and then appointed a curate in Horjul on 24 September 1855,[1][2] where he rented a room at his own expense to found a part-time school to teach the local children catechism, reading, and writing.

[2] At the end of the 1857 school year, Franc Serafin Metelko requested retirement as chair for Slovene at the Ljubljana Lyceum, and Bishop Wolf recommended Marn as his successor.

"[2] At the boarding school in Ljubljana, Marn was a favorite of the institution's head, Janez Zlatoust Pogačar, whom he assisted with translations and corrections for his Slovenski verski časopis (Slovene Religious Newspaper).

His first publications appeared in 1849 in Ivan Navratil's children's magazine Vedež (Knowledge) as short articles with humorous and serious content: "Blagosrčni mladenič" (The Good-Natured Young Man), "Življenje-leto" (Life: A Year), "Povračilo" (Payment), "Kmet in tatje" (The Farmer and the Thieves), "Skopuha vmori glad pri zakladu" (Hunger Kills a Miser by His Treasure), "Vojak zares junak" (A Soldier, A True Hero), and "Plačilo nehvaležnosti in hudodelstva" (Payment for Ingratitude and Wickedness).

Marn's story "Beseda, ktero je govoril bogoslov svojim vrstnikom" (The Word the Theologian Told his Classmates), written in 1853, typifies the orientation that he maintained all his life.

From 1863 until his death he published the newsletter Jezičnik (The Prattler), a supplement to Bishop Wolf's Učiteljski tovariš (Teacher's Friend), for which he wrote linguistics articles, overviews of Slovene literature, and extensive biographical, bibliographical, and literary-history articles about Metelko, Kopitar, and Janez Bleiweis;[1] after 1883 these works appeared under the joint title Knjiga slovenska (The Slovene Book).