The formation of the Duchy of Nassau was directly linked to the rise of Napoleon I to temporary power over most of Europe. In the course of the territorial consolidation of 1803, the Princes Friedrich August zu Nassau-Usingen and Friedrich Wilhelm zu Nassau-Weilburg succeeded in consolidating their territory and, as members of the Confederation of the Rhine, further expanding their rule from 1806. In this year, the foundation of an "indivisible and sovereign state and duchy" was completed.

In terms of foreign policy, the main concern was to maintain independence. Any occasion that could jeopardize the country's existence had to be avoided. The Nassau rulers therefore fulfilled their obligations under the Treaty of the Confederation of the Rhine with particular zeal, especially their duty to provide soldiers for Napoleon's wars. Of a total of 5,628 men, around two percent of the duchy's population, which Nassau sent to the war on the Iberian Peninsula at Napoleon's request, 3,994 did not see their homeland again, a loss rate of 62%. After Napoleon's defeats in Russia and at the Battle of Leipzig, the dukes switched to the side of his opponents in 1813, thus saving the continued existence of the duchy even after the reorganization of Europe at the Congress of Vienna in 1815.

After its final territorial demarcation following an exchange of territory with Prussia in 1815/1816, the Duchy of Nassau covered around 4,700 ^km2 with 286,000 inhabitants, making it the 12th largest of the 39 states of the German Confederation. The economic structure was rural and agrarian; it was one of the poorer regions in Germany. The government's most urgent task was to merge this territorial conglomerate into a unified economic area and into social and cultural unity.

Minister Baron Ernst Marschall von Bieberstein and his close colleague, Government President Karl Friedrich Justus Emil von Ibell, who was particularly committed to the insights of the Enlightenment and national economics, the ideas of the constitutional state and civil equality under the law, were primarily responsible for the internal reorganization of the state. The central aim of the reform was the liberation of the individual from the corporate ties of the corporative society, the reduction of the privileges of the nobility and the equality of all citizens before the law. With this package of reforms, the Duchy of Nassau acquired the reputation of a small German model state in the first two decades of the 19th century and a high degree of popularity among its own population.

In 1820, however, this reform era came to an end and the Nassau government joined Metternich's reactionary policies and became the epitome of a superfluous small state, mostly poorly governed by its monarch using police state methods. The targeted intimidation of any democratic opposition through arbitrary arrests, house searches and treason trials became common state practice. Duke Adolph zu Nassau (r. 1839-66) in particular saw his supposed divine right to rule as the sole political authority.

The feudalistic self-image of the Nassau rulers was expressed, among other things, in the equation of state estates with princely private property. This so-called Domanialvermögen was an estate totaling around 11.5 % of the land area, including mines and mineral springs, the majority of which had only become Nassau property after the Reichsdeputationshauptschluss. In neighboring Prussia and in Hesse-Darmstadt, such "domain fiscus" was considered state property, and the needs of the respective courts were financed via the state budget as a so-called civil list. In the Duchy of Nassau, on the other hand, these assets had to be used exclusively for the court and the private needs of the ruling family.

Another point of conflict was the question of joining the German Customs Union and Nassau's temporary trade war against Prussia. Finally, Duke Adolph's completely excessive passion for hunting also turned large sections of the rural population against him, who had to serve him for 50 to 80 days a year as (until 1848 unpaid) beaters and also had to tolerate the fact that the far excessive density of game kept in large parts of the country at the instigation of the court for the duke's hunting successes caused noticeable damage to crops.

It is therefore not surprising that the German Revolution of 1848 began in Nassau and that around 30,000 protesters, mainly farmers, gathered in Wiesbaden on March 4, 1848 for the first major revolutionary demonstration in Germany. Under the pressure of the revolution of 1848, Duke Adolph did indeed give in to the liberal demands, but broke his public word and carried out a reactionary U-turn as soon as the changed balance of power allowed him to do so. He lost the last of his popular support when, in the conflict between Prussia and Austria, he disregarded the decision of the people's representatives in favor of Nassau's neutrality and marched against Prussia with the military. As a result, he was one of the losers of the War of 1866.

On July 18, 1866, Prussian troops marched into Nassau without a fight. On July 31, the Prussian civil commissioner Gustav von Diest received a petition from 44 well-known liberals and businessmen from the state who pleaded for the annexation of the duchy by Prussia. In none of the German territories annexed by Prussia in 1866 was there as little opposition to this from the population as in the former Duchy of Nassau. The incorporation of the Duchy of Nassau into the Kingdom of Prussia took place on October 3, 1866.