Mackay lived in Berlin from 1896 onwards, and became a friend of scientist and Gemeinschaft der Eigenen co-founder Benedict Friedlaender.

Mackay died in Stahnsdorf on May 16, 1933, ten days after the Nazi book burnings at the Institut für Sexualwissenschaft. Adolf Hitler had become Reichskanzler on January 30, 1933, and all activities of the German homosexual emancipation movement soon ceased. Allegations that Mackay's death may have been a suicide have been disputed:

Using the pseudonym Sagitta, Mackay wrote a series of works for pederastic emancipation, titled Die Bücher der namenlosen Liebe (Books of the Nameless Love). This series was conceived in 1905 and completed in 1913 and included the Fenny Skaller, a story of a pederast. thamyris 02 Under the same pseudonym he also published fiction, such as Holland (1924) and a pederastic novel of the Berlin boy-bars, Der Puppenjunge (The Hustler) (1926).

From 1906, the writings and theories of Mackay had a significant influence on Adolf Brand's organisation Gemeinschaft der Eigenen. Mackay was also a key populariser of the work of Max Stirner (1806—1856) outside Germany, writing a biography of the philosopher which also added greatly to the understanding of the work of Friedrich Nietzsche in the English-speaking world.

Richard Strauss's well-known songs from his Vier Lieder (Op. 27), a wedding gift to his wife, include settings to music of two of Mackay's poems: "Morgen!" and "Heimliche Aufforderung". Other uses of Mackay's poems by Strauss include "Verführung" (Op. 33 No. 1) and "In der Campagna" (Op. 41 No. 2).