The city hall, first documented in 1109, with a stone relief of St. Martin, the town's patron saint, is the oldest in Germany still in use for its original purpose.

Many houses in the town center, notably around the market square, date from the 15th to 17th centuries and have been carefully maintained or restored.

Boniface also established the first bishopric in Germany outside the boundaries of the old Roman Empire on a hill (Büraburg) across the Eder river, where a Frankish fortress and town provided protection, but after the death of Witta, its first and only bishop, in 747 the bishopric was incorporated into the diocese (later archdiocese) of Mainz by Lullus, the disciple and successor of Boniface as archbishop of Mainz.

The monastery was converted into a college of secular canons (Chorherrenstift) in 1005, its members no longer living in monastic union and simplicity, but maintaining their own, and generally rather well-to-do, households in town in the vicinity of the church.

Several imposing stone residences (Curias) built by wealthy canons during the 14th century survive to this day in the old part of the town.

In 1079 Fritzlar ceased to be a crown possession when it was given to the archbishop of Mainz by Emperor Henry IV in the aftermath of his submission to the Pope at Canossa.

Located in the border area between Frankish and Saxon territories and, following Martin Luther's Reformation, a Roman Catholic enclave owned by the Archbishop of Mainz in the midst of Protestant Hesse, the town was frequently embattled, by Saxons and Franks, by Protestant and Catholic princes, and repeatedly sacked and rebuilt.

A papal legate was not able to arrange an end to the dispute, and in early 1079 an army of Saxons, partisans of Rudolf, attacked Henry in Fritzlar.

It was the site of the imperial synod of 1118 at which the papal interdict of Henry V, who again had opposed the pope on the matter of investiture of bishops, was announced and ratified and where Saint Norbert of Xanten, founder of the order of the Premonstratensians (Norbertines) and later archbishop of Magdeburg, successfully defended himself against charges of heresy.

This second basilica was radically reconstructed between 1180 and 1200, essentially in the form in which it is still found today, although a number of smaller additions and alterations have been made throughout the centuries since then.

The Thirty Year War (1618–1648) inflicted serious damage on Fritzlar and the neighboring villages, culminating with an outbreak of the black plague.

During the Seven Years' War (1756–1763) the town was occupied by French troops and parts of its fortifications were destroyed, along with the vineyards on the steep slope above the Eder river.

Between 1933 and 1945, the systematic marginalization, segregation, expulsion, and murder of the Jewish community of Fritzlar is documented in "Der antijüdische Rassenwahn Hitlers, Juden in Fritzlar und seinen Ortsteilen und ihre wenigen Freunde: Erweiterte Auflage Aug 15, 2014" by Paulgerhard Lohmann In 1974, the three districts of Fritzlar-Homberg, Melsungen and Ziegenhain were combined into the new district Schwalm-Eder, with its administrative seat in Homberg (Efze).

As of the last municipal election held on 31 March 2021, the seats are apportioned thus: The town executive (Magistrat) consists of 10 members and the mayor.