The era of the Teutonic Order is a significant milestone in the long history of the island of Mainau. A time, whose traces are still clearly visible today, above all with the prominent Baroque building complex of castle and church. The commanders of the ecclesiastical order of knights, founded at the time of the crusades, prevailed as the administrators of the settlement of this order on the island, until 1806, so over 500 years. However, after bestowal, Mainau including all the accompanying lands on the mainland, initially went to the settlement of the Teutonic Order in Castle Sandegg near Salenstein, in the neighbouring canton of Thurgau. In 1272, the commandry of Sandegg was dissolved and relocated to Mainau. There, the castle was expanded, the acquisition of goods by the Teutonic order was extended to the north, so that the commandry of Mainau ultimately developed into the most wealthy settlement of the province of the order Alsace-Burgundy. 

During the Thirty Years War (1618–48), Sweden also took over the rule of Mainau for two years. If the island was initially a safe place of refuge, the Swedish troops still succeeded in taking it over in 1647. Only in the end phase of the war was Mainau secured by a closed earthwork system, from which only a few remains are recognisable today.

After the Westphalian Peace in 1649, the Swedes moved out, but not without having raided and plundered the island. Only the so called “Swedish cross” was apparently too heavy for them, which is why, according to legend, the crucifixion group cast in bronze in Constance in 1577, was left in the shallow water between the island and the mainland. Later, it was erected there, and today greets visitors to Mainau as the “Swedish cross”.

The order commandry of Mainau took a long time to recover from the consequences of war. Yet, it could be seen as the start of a new era, that the Provincial Superior of the order province of Alsace-Burgundy issued the authorisation to build a new church on the island. The contract was awarded to the architect of the order Johann Caspar Bagnato (1696-1757), who started to build the Baroque church in 1732. In 1739, the castle church of the virgin Mary was inaugurated, and in the same year, Bagnato also received the contract to build a castle on the island. The castle previously used as the seat of the commander, was demolished for the new building. After a seven-year building period, the castle of the Teutonic Order was closed in 1746. The magnificent coats of arms on the gables of the middle wing, are still reminiscent of the builder, even today. 

In the course of secularisation, the Teutonic Order Commandry of Mainau was dissolved in 1806, and became part of the newly founded Grand Duchy of Baden, with all possessions.

After the death of Grand Duke Frederick I in 1907, Mainau went to his son, Grand Duke Frederick II. He bequeathed the island to his sister Viktoria, Queen of Sweden, which is how the island of Mainau became the property of the Swedish royal family in 1928. After the death of Queen Viktoria, the island went to her son Prince Wilhelm of Sweden in 1930, who transferred the management of the inheritance to his son Lennart in 1932, who was just 23 at the time. In the same year, Lennart Bernadotte moved back from Sweden to Main, after he lost all titles and claims to inheritance to the Swedish royal family, after marrying the commoner Karin Nissvandt, and made the island into a new residence for him and his family. Lennart Bernadotte began to convert the neglected island into a park again, little by little and with a great deal of effort, soon opened Mainau to visitors, and the “Schwedenschenke” restaurant was opened to cater for them. In the years before the Second World War, Mainau profited not insignificantly from state managed tourism, the “Kraft durch Freude” (Strength through Joy) travels, which brought thousands of visitors to the island.

War and post war period

Even before the start of the war in 1939, Lennart Bernadotte left the island He spent the time until the end of the Second World War with his family in Sweden. A married couple were appointed as administrators, to manage the building, park, garden and also the visitors to Mainau, and were able to take care of the facilities and the business, until the administrator was called up to the German Army in 1942. In this situation, a request from the Todt Organisation was convenient for Prince Wilhelm of Sweden and his son Lennart Bernadotte. The structural engineering organisation of the Ministry of Defence wanted to rent Mainau for the duration of the war, and set up a rest home for officers and industrialists. The rental contract came to pass, and from mid 1943, Mainau was managed by the Todt Organisation. The rest home was to be set up in the castle and archway building, and three additional wooden barracks were built to accommodate the staff. A water pipeline was also laid from the Constance district of Egg to the island. So, everything was prepared, yet high ranking employees of the Todt Organisation no longer had the opportunity to recover on Mainau.

Instead, shortly before the end of the war, the Foreign Office gave the island to French collaborators, who rallied around the head of the extreme right wing “Parti Populaire Français” (PPF), Jacques Doriot. From the south of Germany, the group planned to drive the Gaullists and communists out of France, which is why Doriot proclaimed a French liberation committee on the island of Mainau at the beginning of 1945. However, this German – French collaboration ended in February 1945, with the death of Doriot, in a low flying aeroplane bomb attack near to Mengen. As a result, his followers fled Mainau.

On 26 April 1945, a new era began for Constance and the surrounding area: The region became part of the French occupation zone. Mid May 1945, the islands of Mainau and Reichenau were cleared. The islands were selected for the accommodation and recovery of French prisoners liberated from the Dachau concentration camp. A total of several thousand prisoners were brought to Lake Constance, most of which to Reichenau. The castle on Mainau and the barracks built by the Todt Organisation now stood empty. They now served as accommodation, above all for seriously ill and particularly weakened former concentration camp prisoners, who were treated, cared for and supervised by French doctors, and French and German nurses, according to the latest perspectives. Yet for 33 of them, it was too late to help them. In spite of good care, they died during their stay on Mainau, and were initially buried in a temporary grave yard on the east bank of the island. After his first visit to the island after the end of the war, Lennart Bernadotte arranged for the dead to be buried in the Constance main cemetery. Between 1947 and 1949, the human remains were then sent to France.

In September 1945, the military hospital on Mainau was dissolved and the former concentration camp prisoners who had regained their strength were taken to France. The French occupying troops also ultimately left Mainau again. When Lennart Bernadotte came back to the island for the first time in January 1946, by his own account, he found a deteriorated facility and a mostly empty castle. A long conflict arose with the French authorities regarding the regulation of the damages, and the replacement of missing furnishings. The construction and repair works in the part and garden were difficult, however Mainau was soon open to visitors again.

In order to open a “window to the world” for young people after the destruction of the war, even in 1946, Lennart Bernadotte offered meetings and leisure time for young people on Mainau, in the framework of the Young Men's Christian Association (YMCA) – today the Christian Young People's Association – under Swedish management. The participants were initially housed in the barracks built by the Todt Organisation, which were torn down in 1951. In these difficult post war years, the care sometimes came from Sweden and Switzerland. The aim was to allow young people war-damaged young people to recover, and have international meetings with young people from other countries.

This institution developed into the “International Institute Castle Mainau”, which began operation in 1949, under the sponsorship of the YMCA world association, in a wing of the castle, and offered courses, seminars and workshops on youth issues in various areas. By the time the institute closed at the end of 1968, around 20,000 participants from 40 countries had attended the events at Mainau Palace.