Lahner's mother died in childbirth and he became an orphan at the age of seven when his father was killed in an accident. Placed in the care of a bishop guardian, he was sent to boarding school to begin his training as an engineer. This choice of a career was his guardian's and it was not until after his graduation in 1910 that Lahner was able to pursue his true passion: oil painting.

Lahner's enthusiasm for painting seems to have stemmed from a boyhood incident when he came upon a man restoring a crucifix near his village. The artisan offered the curious boy a tube of green paint. This event sparked the young boy's interest in art and led to what became a lifelong vocation.

Free from his guardian's influence Lahner abandoned his engineering career in 1915 and enrolled in the School of Fine Arts in Budapest where he studied under the masters Vaszary and Kochine. During this early stage of artistic development almost all of his works were landscapes of tiny villages and wooded hills.

Immediately following World War I much of Hungary was sacked by neighboring Romania and the new Soviet Republic. During the subsequent "Red Terror" and "White Terror" thousands were either jailed or killed. In this harshly repressive atmosphere many artists and intellectuals, including Lahner, were forced to flee their homeland or chose to emigrate.