Antal Molnár's parents intended him to be a lawyer; despite this, he chose a musical career. It was Zoltán Kodály's personality and the harmonics studies he had with him that led him in that direction. As a teenager he learned the piano and the violin, then, between 1907 and 1910 he completed the composition class at the Music Academy under the direction of Viktor Herzfeld. At that time he had not yet committed himself to any one particular sphere of music, since he was also attracted to literature and writing as a vocation. "Till I was about twenty I did not know whether I would be a writer or a composer, or a violinist or an aesthete. Everything was in turmoil inside me. I wrote many short stories and had two books of poems published", he said later about these years. His career as an instrumentalist also began well: from 1910 for three years he played the viola in the Waldbauer-Kerpely String Quartet, then between 1917 and 1919 he was a regular participant in Ernő Dohnányi and Jenő Hubay's joint chamber music evenings. In the 1910's he made the acquaintance of the writers of the famous literary journal, Nyugat, and took part, as composer and as piano accompanist, in the Nyugat's matinees, as well as publishing articles on music in the journal. At the instigation of Bartók and Kodály, he collected folk songs, in 1910 in Transylvania and in 1912 in today's Slovakia.
His interest in musicology developed in the 1920's: he published analytic studies of Bartók's works, then, with the monograph, "The history of European music up to 1750" (1920) he began his comprehensive studies of musical history. Though he was considered to have a promising talent for composition, at this point he stopped composing. As he told Ferenc Bónis in a television interview in 1975, "…if one, for whatever reason, permits oneself to be persuaded to write books instead of composing, this fact in itself is proof that one's talent as a composer is of a lesser order than as a writer." His creative and performing ambitions and his interest in new music were from this time on realised in articles and studies on the art of his contemporaries, particularly Bartók and Kodály. During the more than five decades that Antal Molnár devoted to writing about music, he became one of the fathers of modern Hungarian musicology. In his books of fundamental significance dealing comprehensively with musical history and aesthetics, books which are still used today, he employed also the results of European general aesthetics and philosophy. His momentous life's work dealt also – for the first time in Hungary –with the sociological aspects of musical history, examined the interconnections between physics, psychology, education and music, and the general phenomena of musical culture.
He taught theoretical and practical subjects, music history, music aesthetics and music theory alike at the Music Academy for forty years, from 1918 onwards. He wrote some of his books to provide a background for his teaching. After his retirement, he still published a number of books; his collected reminiscences are important documents of twentieth century Hungarian musical history.
Antal Molnár was among the first to recognise that in laying the foundations for the musical culture of a country, besides scientific work and teaching, high-level popularising of music is also indispensable, and that this task belongs primarily to music historians. He professed that even the most complex professional knowledge can be conveyed in a form that is comprehensible for the lay reader if the writer discusses musical phenomena factually, but in an enjoyable style and with a personal commitment towards the works in question. In his popularising articles and radio talks, he endeavoured not merely to convey information but to bring the audience to come to realise the possibility open to everyone of experiencing the beauty of music and music making.