August was the first born and only surviving child of Michael and Maria Kubizek. His sisters Maria, Therese and Karoline died in early childhood. Kubizek later wrote that this was a striking parallel between his own life and that of Adolf Hitler, whose mother had lost four children prematurely. As the surviving sons of grief stricken mothers, August and Adolf could not help but feel they had been spared or "chosen" by fate.

Kubizek and Hitler first met while competing for standing room in the Landestheater in Linz, Austria. Because of their shared passion for the operas of Richard Wagner they quickly became close friends and later roommates in Vienna while both sought admission into college. The two shared a small room in Stumpergasse 29/2 door 17 in the sixth district of Vienna from 22 February to early July 1908.

As the only son of a self-employed upholsterer, August was expected to someday take over his father's business, but he secretly harboured dreams of becoming a conductor. With Adolf's encouragement, he devoted more and more of his time to this passion, completing all the musical training available to him in Linz. However, to achieve his goal, he would require higher education in music which was only offered in Vienna.

It was Adolf Hitler who, at the age of eighteen, successfully persuaded Kubizek's father to let his son go to the metropolis to attend the conservatory. This, Kubizek wrote, changed the course of his life for good.

He was immediately accepted into the Vienna Conservatory where he quickly made a name for himself. Hitler, however, was twice denied entrance into Vienna's art academy, a fact which he kept hidden from his friend for some time. In 1908 Hitler abruptly broke off the friendship and drifted into homelessness. Kubizek completed his studies in 1912 and was hired as conductor of the orchestra in Marburg on the Drau, Austria (called Maribor in Slovenia after 1918). He was later offered a position at the Stadttheater in Klagenfurt, but this job and his musical career were cut short by the beginning of World War I. Before leaving for the front he married Anna Funke (7 October 1887 — 4 October 1976), a violinist from Vienna with whom he had three sons: Augustin, Karl Maria and Rudolf.

From August 1914 until November 1918 Kubizek served as a reservist in Regiment 2 of the Austro-Hungarian Infantry. In the Carpathian winter campaign of 1915, he was wounded at Eperjes in Hungary (now Pre?ov in Slovakia) and later evacuated to Budapest in an ambulance train. After months of convalescence, he returned to the front and was attached to a mechanised corps in Vienna. After the war Kubizek accepted a position as an official in the municipal council of Eferding, Upper Austria and music became his hobby.

After seeing Hitler on the front page of the Münchner Illustrierte (circa 1920) Kubizek followed his friend's career with some interest, although he did not attempt to contact him until 1933 when he wrote to congratulate him on having become Chancellor of Germany. Six months later Kubizek received an unexpected reply from Hitler, who wrote to his old friend "Gustl" saying, "I should be very glad... to revive once more with you those memories of the best years of my life." Thirty years after Hitler had broken off contact with Kubizek the two friends were reunited on 9 April 1938 during one of Hitler's visits in Linz. The two spoke for over an hour at the Hotel Weinzinger and Hitler offered Kubizek the conductorship of an orchestra, which Kubizek politely refused. Upon learning of his friend's three sons Hitler did, however, insist on financing their educations at the Anton Bruckner Conservatory in Linz. Hitler later invited Kubizek to attend the Bayreuth festival as his guest in 1939 and again in 1940, experiences described by Kubizek as "the happiest hours of my earthly existence".

In 1938, Kubizek was hired by the Nazi party to write two short propaganda booklets called Reminiscences about his youth with Hitler. In one episode Kubizek said Hitler had a great love for a girl named "Stefanie" and wrote her many love poems but never sent them. Hitler biographer John Toland noted that when Stefanie learned she had been an early object of Hitler's affection, she was stunned.

Kubizek saw Hitler for the last time on 23 July 1940, although as late as 1944 Hitler sent Kubizek's mother a food basket for her 80th birthday. His friend told him: "This war will set us back many years in our building programme. It is a tragedy. I did not become Chancellor of the Greater German Reich to fight wars." The Führer was speaking after the successful campaigns in Poland and France.

When the tide began to turn against Hitler's favour, Kubizek, who had avoided politics all his life, became a member of the NSDAP in 1942 as a gesture of loyalty to his friend.

In December 1945, Kubizek gathered the collection of postcards and other keepsakes given to him by Hitler during their youth and concealed them carefully in the basement of his house in Eferding. He was arrested shortly afterwards and held at Glasenbach, where he was imprisoned and interrogated by the U.S. Army Criminal Investigation Command for 16 months. His home was searched, but the Hitler correspondence and drawings were not found. He was released on 8 April 1947.

In 1951, Kubizek, who had rejected other post-war offers for his memoirs, agreed to publish "Adolf Hitler, mein Jugendfreund" ("The Young Hitler I Knew") through the Leopold Stocker Verlag. The original manuscript was 293 pages long and included several pictures, many of which showed postcards and sketches given to the author by the young Hitler between the years 1906 and 1908. The book is divided into three parts and consists of a prologue, 24 chapters and an epilogue.

It caused a stir when it was released in 1953 and was later translated into several languages. In the epilogue Kubizek wrote, "Even though I, a fundamentally unpolitical individual, had always kept aloof from the political events of the period which ended forever in 1945, nevertheless no power on earth could compel me to deny my friendship with Adolf Hitler."

Kubizek's second wife and widow Pauline (1906—2001) was credited with having provided the Stocker Verlag with additional photographs for the book's fourth edition in 1975.

On 8 January 1956 Kubizek was named the first honorary member of the Musikverein in Eferding. He died on 23 October 1956, aged 68, in Linz and is buried in Eferding, Upper Austria.