The construction of St.Mary's started in 1284, when the city of Flensburg was founded. The nave of the three-winged gothic church was extended to the east in the 15th century, and, after two years of construction, a Neo-Gothic steeple was added in 1880. Visitors can still see a 1589 sandstone sculpture of Our Lady on the new steeple. There are lovely mural paintings from the 15th century inside the church that visitors can admire when walking around the property. The altar dates back to 1598 and is considered to be the most valuable part of the church. The church is also considered to be one of the most important late Renaissance works of art in Schleswig-Holstein. Other impressive pieces are the carved wooden pulpit (1579), the bronze font (1591) by Michael Dibler and the beautiful epitaphs from the 16th century, some of which are by Jan van Enum. The modern stained glass windows were designed by Käte Lassen in 1948.