His best known work, Indenfor Murene, premiered in 1912 at the Royal Danish Theatre, directed by the author.

[1] The play centers around a wealthy, loving, but conservative Jewish family whose only daughter breaks away from tradition by attending lectures at the university and secretly becoming engaged to her teacher, a gentile.

[2] Late in his career, Nathansen wrote a number of biographies, notably one of Georg Brandes (1929).

In October 1943, when the Nazis attempted to round up the Danish Jews, Nathansen fled to Sweden.

A bust of Nathansen stands in the small garden complex Digterlunden next to the Town Hall Square in Frederiksberg.