He led the Bulgarian delegation to the First Linguistic Congress in Prague in 1935.

In 1929, his "History of the Bulgarian language" was published in German under the review of Max Vasmer.

Previously edited the special part in the second and third volume of the first three-volume story in Bulgarian by Benyo Tsonev.

His writings are fundamental to historical comparativism, because Old Bulgarian is the fourth classical and medieval literary language (see trilingual heresy).

Stefan Mladenov made a special contribution to clarifying the historical influence of the Bulgarian language and to the emergence of languages from the Balkan Linguistic Union.