His reign found him mostly occupied with a successful civil war against his half-brother, Chilperic.

When Clotaire I died in 561, his kingdom was divided, in accordance with Frankish custom, among his four sons: Sigebert became king of the northeastern portion, known as Austrasia, with its capital at Rheims, to which he added further territory on the death of his brother, Charibert I, in 567 or 568; Charibert himself had received the kingdom centred on Paris; Guntram received the Kingdom of Burgundy with its capital at Orléans; and the youngest son, the aforementioned Chilperic, received Soissons, which became Neustria when he received his share of Charibert's kingdom.

Incursions by the Avars, a fierce nomadic tribe related to the Huns, caused Sigebert to move his capital from Rheims to Metz.

Probably spurred by his wife Brunhilda's anger at her sister's murder, Sigebert sought revenge.

But at Sigebert's moment of triumph, when he had just been declared king by Chilperic's subjects at Vitry-en-Artois, he was struck down by two assassins working for Fredegund.