After failing as photographer and because of his psychosis, a severe mental disorder in which thought and emotions are so impaired that contact is lost with reality, he became a professional chess master. In 1916, he was hospitalized for his condition, was released, but was hospitalized again after a relapse.
The outbreak of the First World War ruined his life and was probably the cause of his illness and the reason why he was deported back to Switzerland from Germany. Back in Switzerland, he was heavily involved with art and chess composition. He was the first Swiss composer to compose over 150 studies which often resembled over the board positions. Eighty-three of his compositions were composed together with the Dutch problemist Johannes Willem Keemink. Almost nothing is known about Keemink except that in 1920 he played a match against Euwe and his only studies were composed with Fahrni. The two published a book of endgame studies in 1928
According to Chessmetrics, Fahrni's highest rating was in 1906 with an assigned rating of 2557. By 1917 his rating had dropped a bit to 2510, but he was ranked at number 20 in the world. For 1917, the top rated players were Lasker, Capablanca, Marshall, Schlechter and Tarrasch.
His best ever tournament performance came in 1911 at San Remo where he finished 1st, ahead of Levitsky, Przepiorka, Gunsberg, Kostic, Forgacs and Reti. Fahrni played several matches. In 1907, he lost to Spielmann (+3 –5 =2). In 1908, he drew with Alekhine (+1 –1 =1).
The same year he defeated Salwe (+3 –1 =1). In 1910, he lost to Spielmann (+3 –4 =4). In 1912, he defeated von Bardeleben (+3 –0 =1). In 1914, he drew with Leonhardt (+1 –1 =0). In 1916, he drew with Selesnev (+2 –2 =2). In 1917, he lost to Teichmann (+0 –2 =2).
During his career he also authored three chess books, two on the endgame and his monograph on 1.e4 Nf6 was the first time it was referred to as Alekhine's Defense. Although Wikipedia and other sources say that he was joint Swiss champion with Oscar Corrodi in 1892, according to the websites of both the Swiss chess federation and ARVES (an endgame studies site) it was actually his brother Paul Fahrni.