Two towns with the same name of Mezhyrich belonged to the princes of Ostroh. The town of Mezhyrich Koretskyi, which got its name from the neighboring Korets, was of no particular significance to the princes despite Kostiantyn Ivanovych used the town as his residence in the late fifteenth century. Mezhyrich, which was located near Ostroh, did not get the princes’ attention for a long time as well, although they acquired this village in the 14th century.
Only at the beginning of the sixteenth century, the Grand Hetman of Lithuania Kostiantyn Ivanovych Ostrozkyi commenced its construction and development. The first stone was laid in the foundation of the Trinity Church. The prince built it as a tribute to the victory over the Muscovites in the battle of Orsha. It was part of his vow to build temples in Vilnius and Ostroh as gratitude for the victory over the Moscow voivode. Since the temple additionally performed defensive functions, loopholes in the walls were considered.
Prince Yanush Ostrozkyi was involved in the development of the town even more. In 1605, he received the privilege of Magdeburg law and the right to hold trade fairs in Mezhyrich. In a few years, the town was fortified with an earthen rampart and bastions. The monastery, which Yanush built for the monks of the Order of St. Francis, was the most fortified. As well, for the needs of the monks, the prince handed over the Orthodox church, which was soon converted into a Catholic church. The monastery had remained Catholic until 1866. Then, it passed to the Orthodox community.
Next to the monastery, Prince Yanush built a large wooden palace with a kitchen with a banquet hall. However, in 1648, the Cossacks burned down the palace, leaving only a stove with the Renaissance style upper part of a chimney.
The miraculous ancestral icon of the Ostroh princes, the Mother of God of Mezhyrich, has been preserved and still belongs to the monastery. Grand Hetman Konstantin Ivanovich Ostrozky commissioned the icon. The icon of the Virgin and Child was an expression of gratitude for the prince's victory in the battle of Orsha. Although the icon was painted in the Byzantine style, the background got all the hallmarks of the Western European Gothic tradition. In 1779, Pope Benedict the 14th authorized the suffragan of Lutsk Frantsysk Komarnytskyi to crown the Virgin of Mezhyrich and sent a golden crown for this purpose. Later, in the nineteenth century, the Russian Empress Mariia Oleksandrivna presented emeralds for this crown.