The history of Lahnstein dates back to the first beginnings of settlement of the Rhine valley. Numerous archaeological finds bear witness of human life in Lahnstein and the surrounding area since the younger Stone Age. In the years 81-96 AD, the Romans erected a defence line on the right side of the Rhine, the Limes. At the mouth of the Lahn, a burgus was built in the third century AD to protect the Rhine line, which also gave the town its name.
In Frankish times, Lahnstein was Salland (= crown land) that belonged to the King and was administered by a manor farm, the Salhof. Around the year 900, Oberlahnstein came into the possession of the Archbishopric of Mainz, as confirmed by a document issued in 977 by Emperor Otto II to Archbishop Willigis of Mainz. In 1018, the royal manor Niederlahnstein passed to the Archbishopric of Trier when Emperor Heinrich II donated the royal court of Koblenz to the Archbishopric. Thereby, the Lahn became the border. In order to protect their land, the Archbishops of Mainz built Burg Lahneck around 1240. In 1324, Oberlahnstein was granted town status – and thus the same liberties as Frankfurt – by King Louis the Bavarian, and in the following decades, the town was strongly fortified and girdled with a twofold town wall with a moat and sixteen towers. In 1332, Louis the Bavarian also gave Prince-elector Balduin of Trier town privileges for Niederlahnstein, which was protected by several well-fortified borderland farms, ramparts, moats and gates.
The central location (Oberlahnstein belonged to Mainz, Niederlahnstein to Trier, Rhens to Cologne, Marksburg to the Palatinate) made Lahnstein an ideal gathering place and thus saw several important kings' visits throughout the Middle Ages.
Due to its location on the Rhine, Lahnstein was involved in numerous wars and was conquered several times. When Napoleon disbanded the electoral states of the Holy Roman Empire, the Lahn was no longer the frontier, and the towns of Ober- and Niederlahnstein passed to the Duchy of Nassau which was annexed by Prussia in 1866. Now both towns belonged to the Prussian province Hesse-Nassau. Niederlahnstein received town privileges in 1885 and thus obtained the town liberties as suggested in an imperial document from 1332.
After the Second World War, both towns became part of the newly-founded federal state of Rhineland-Palatinate in 1946. As part of the administrative reform of the 7th June 1969, the towns Ober- and Niederlahnstein were consolidated into the town Lahnstein.