[1] In early 1884, he was invited to a personal meeting with Pope Leo XIII in Rome, who confirmed the appointment of the young bishop of Apostolic Vicariate for Catholic Bulgarians in Macedonia, based in Thessaloniki.

Thanks to his energy and the financial backing he received from the Church, Mladenov was able to provide several initiatives with grants, found several schools in his diocese and started working on a highschool in Kilkis, which he wasn't able to finish.

In 1894, financial scandals and affairs caused conflicts between several Catholic factions, and eventually led to his departure from the Episcopal department at the end of October 1894.

[2] In early December 1894, Lazar Mladenov asked the Holy Synod in Sofia to be accepted into the Eastern Orthodox Church.

After 18 years in office, he wrote a book on the history of Bulgaria based on unpublished documents from the jurisdiction of his library.