The first church was dedicated to Mother Mary and St. George. The Romanesque basilica was 25 meters long and 16 meters wide, has two absident towers in the east, which are about 30 meters high. Apart from these, two Romanesque portals are preserved from the original building. The biggest one represents John, Mary, Christ, Peter and George. In 1323 the church received a relic of St. Leonard. After 1425 the late Gothic choir was built, which is one of the most beautiful of the Middle Rhine area of Gothic period. In 1508 the Holzhausen family had a small choir built next to the church, which was later mentioned in many guidebooks because of its hanging vault.
The history of St. Leonhard is closely connected with that of the church of the former Carmelite monastery (around 1250 to 1803), today the main site of the Frankfurt Archaeological Museum. The two medieval churches in the west of Frankfurt's former old town, a good 100 meters apart, are connected by more than just their spatial proximity and their building history, which dates back to the 13th century. After the dissolution of the monastery in 1803, parts of its inventory were transferred to St. Leonhard. The parish still uses the liturgical equipment, two confessionals and the pews from the Carmelite church.
In 1927-1928 and 2009-2017, the medieval furnishings and unique finds were rediscovered within the Leonhard Church. The excavations revealed, among other things, outstanding examples of late medieval sculptures in the backfill rubble of the early 19th century.
