Hiedler was born to Stefan Hiedler (1762–1829) and Anna Maria Göschl (1760–1854), in Spital – a part of Weitra – Austria, and made his living as a wandering journeyman miller. In 1824, he married his first wife but she died in childbirth five months later.
He married Maria Anna Schicklgruber in 1842, and became the legal stepfather to her illegitimate five-year-old son, Alois. It was later claimed Johann Georg had fathered Alois prior to his marriage to Maria, although Alois had been declared illegitimate on his birth certificate and baptism papers; the claim that Johann Georg was the true father of Alois was not made after the marriage of Maria and Johann Georg, or, indeed, even during the lifetime of either of them.
In 1877, twenty years after the death of Johann Georg and almost thirty years after the death of Maria, Alois was legally declared to have been Johann Georg's son. Johann Georg's younger brother, Johann Nepomuk Hiedler, engineered the plan to change Alois' surname to "Hitler" and to have Johann Georg declared the biological father of Alois in 1876. Johann Nepomuk collected three "witnesses" (his son-in-law and two others), who testified before a notary in Weitra that Johann Georg had several times stated in their presence that he was the actual father of Alois and wanted to make Alois his legitimate son and heir. The parish priest in Doellersheim, where the original birth certificate of Alois resided, altered the birth register. Alois was thirty-nine years old at the time and was well-known in the community as "Alois Shicklgruber."
Accordingly, Johann Georg Hiedler is one of two people most cited by modern historians as having possibly been the actual paternal grandfather of Adolf Hitler. The other one is Johann Nepomuk Hiedler, the younger brother of Johann Georg.
In the 1950s, a claim surfaced that a Jew from Graz by the name of Leopold Frankenberger was Hitler's paternal grandfather, but modern historians have debunked this possibility, as Jews were expelled from Graz in the fifteenth century and were not permitted to return until the 1860s, several decades after Alois was born.