Asther was the son of Anton Asther and Hilda Åkerlund. Although his parents were Swedish, Asther was born in Denmark in Sankt Matteus parish, Copenhagen, where unwed mothers at the time often went to give birth discreetly. Nils spent his first year as a foster child in Limhamn, Sweden, until his parents married in 1898 and he moved in with them in Malmö, where he would grow up. As a young man he moved to Stockholm, where he received acting lessons from Augusta Lindberg. It was through Lindberg that Asther received his first theatrical engagement at Lorensbergsteatern in Gothenburg, and in 1916 Mauritz Stiller cast him in The Wings (Swedish: Vingarne), a gay-themed Swedish silent film from 1916, directed by Mauritz Stiller, based on Herman Bang's 1902 novel Mikaël. In Copenhagen, one of Sweden's best known actors, Aage Hertel of the Royal Danish Theatre, took Asther under his wing. This soon led to a number of film roles in Sweden, Denmark and Germany between 1918 and 1926.