The municipality lies in the middle of the Hunsrück between Simmern and Kastellaun, right on the Schinderhannes-Radweg (cycle path) at a mean elevation of 320 m above sea level. The Külzbach marks the boundary between Keidelheim and the neighbouring village of Kümbdchen.
Sometime between 1330 and 1335, Keidelheim (then Kudelnheim) was first mentioned in a document in the taxation register kept by the Count of Sponheim. The register listed a whole series of taxes and other comital levies, among which were the soul tax, the head tax and tithes.
Another compilation of taxes from about 1400 lists for the first time the village’s inhabitants (it was then called Kudillenheim). The monasterial lordship over Keidelheim ended in 1566 with the dissolution of the Ravengiersburg Augustinian Canonical Foundation. The Duchy of Simmern passed as an Oberamt to the Electorate of the Palatinate. Over the next 120 years, the countryside was heavily stricken with many wars, bad harvests and sicknesses. The villages of Nannhausen, Fronhofen, Biebern, Reich, Wüschheim, Eichkülz and Keidelheim together formed a Schultheißerei. In 1683, Michel Huth from Keidelheim was the Schultheiß.
On 19 October 1794, French troops marched into Simmern, ending the Electorate of the Palatinate rule. Patrimonial hierarchy (Erbuntertänigkeit – an arrangement similar to serfdom, but without the lord’s actual ownership of the subjects), with its compulsory labour for the enfeoffed lord and its tithes was then abolished, but new compulsory labour for the French army weighed even more heavily on the people. The official cession of the Rhine’s left bank to France in 1797 led to administrative restructuring. The nobility lost their holdings, churches and monasteries were expropriated and estates could be acquired by those who had hitherto been their tenants. Keidelheim was grouped with 12 other municipalities into the Mairie (“Mayoralty”) of Simmern in the Canton of Simmern, thereby making it part of the Department of Rhin-et-Moselle, whose seat was at Koblenz. As French citizens, young men were then required to do military service. With Napoleon’s defeat in Russia began a time in which Austrians, Russians, Bavarians and Prussians quartered themselves locally and imposed more compulsory labour on the people. At first, the area was governed by an Austrian-Bavarian administration commission. In an 1813 statistical publication, the following figures were noted for Keidelheim: 114 souls, 25 houses and 25 windows.
On 28 May 1815, the Prussians took over the lands on the Rhine’s left bank, and from the French arrondissement of Simmern arose the district (Kreis) of Simmern. The municipality of Keidelheim passed to the Bürgermeisterei (“Mayoralty”) of Simmern, and is still part of that – in its newer incarnation as a Verbandsgemeinde – even now. Despite progressive Prussian measures in agriculture and road building, bad harvests and a sharp rise in population led to gradual impoverishment. The municipalities had to take measures to save the inhabitants from going hungry. Many citizens emigrated to the United States.