Siegbert III was banished to an Irish monastery by Pepin of Landen. He ends the French Merovingian House; reigned 639-54 (or per "The Lives of the Kings and Queens of France" by Duc de Castries, 638-56). His wife was Immachilde. His brother Clovis was King of Neustria and Burgundy. At this point the era of administration of the Mayors of the Palace begins.

Sigebert was the son of Dagobert I and Ragnetrude. He was made king of Austrasia c. 635 (1) by his father, who moved his capital to Paris, but was prompted by the Austrasian nobility to give them a king according to a Frankish tradition of separate courts for Neustria and Austrasia.

Sigebert III was one of the first rois fainéants ("sluggard kings") of the Merovingian dynasty, who held no real power of his own but was ruled by mayors of the palace. During his minority he was governed by Cunibert, bishop of Cologne, and Duke Adalgisile; then, on Dagobert's death, by Cunibert and Pépin de Landen, the mayor of the palace, who died c. 640. When Sigebert came of age, Pépin's son, Grimoald, mayor of the palace from 642 or 643, exercised full power in king's name. In the early 640s, when a certain Otto was mayor of the palace, Thuringia succeeded in gaining effective autonomy.

The date of Sigebert's death is rather questionable. It occurred on February 1, but the year is uncertain. Most probably, it was 656, but some sources point at 662 or even 650. After his death, his young son, future Dagobert II, was sent off to an Irish monastery by Grimoald, who briefly established his own son, Childebert (adopted by Sigebert when the king was still childless) on the throne.