At the National Theatre Bucharest, Nicolae Massim made his debut as assistant director to Ion Sahighian, and during long and distinguished career with the National Theatre, Nicolae Massim directed many memorable hits such as Othello, The House of Bernarda Alba or "The Glass of Water". In parallel with his activity at the National Theatre of Bucharest, Nicolae Massim was a professor, and for a while the dean of the Stage Directing department, within the Institute of Dramatic Arts and Cinematography (IATC) of Bucharest.
In 1945, together with Lucia Calomeri, an actress, Elena Pătrășcanu and Lena Constante, set design artists, Nicolae Massim founded the world-famous "Țăndărica" Marionette and Puppet Theatre of Bucharest. Recruiting a first nucleus of talented and enthusiastic puppeteers such as Dorina Tănăsescu, Antigona Papazicopol and Elvira Chladek, and using the voices of reputed actors from the National Theatre of Bucharest, Nicolae Massim directed the first marionette show in Romania "With Țăndărica to the Southern Seas". After that, throughout his entire career, Nicolae Massim remained an enthusiastic contributor to the theatre being an artistic mentor of stage director Margareta Niculescu, his former student at IATC, who in 1949 replaced Lucia Calomeri as artistic director of the theatre.