A native of Munich, he was director of the Bavarian State Painting Collections, a position in German arts administration second only to the head of the Berlin museum network.
Buchner's sharp career focus, along with his scholarly expertise and connections, culminated in the directorship of the Bavarian State Painting Collections in July 1932.
The report of the post-war Art Looting Investigation Unit mentioned his "fixed belief in a Greater Germany—whether the Fuehrer be Frederick the Great, William the Second, or Hitler".
Eventually, he purchased confiscated art for his own museum collections directly from the Gestapo, including pieces by Eugène Delacroix and Wilhelm Trübner.
He parted with pieces by Raphael, Rubens, Renoir, and Monet in exchange for artists like Matthias Grünewald (for a panel of questionable authenticity), Hans Thoma, Wolf Huber, and Johann Jakob Zeiller.
Buchner was ordered to lead a search for van Eyck's Ghent Altarpiece, which had been moved from Belgium to unoccupied France for safekeeping from war.
Although the transfer took place with French cooperation, and Buchner testified that the goal of the seizure was only to protect the painting, according to historian Jonathan Petropoulos, it was "[not so] ambiguous ... that the Germans were claiming ownership of the masterpiece".
In May 1945, Buchner had been removed from his position at BSGS, and he would have to go through the denazification process if he wished to work again as a civil servant in the Federal Republic of Germany.
While he had, for example, accepted an honorarium of 30,000 Reichsmarks from Hitler for recovering the Ghent Altarpiece, he was credited with avoiding the most ideologically extreme positions of the Third Reich with respect to the arts.
Supporters emphasized his undisputed expertise, his role in the protection of artworks during World War II, and his abiding interest in rebuilding German culture, including the replacement of damaged museums (respecting their original architecture).
He spent the remainder of his life researching and writing, and died in June 1962 while working on a five-volume study of German panel painting.