Agnes was the daughter of Peter of Savoy and Agnes of Aquitaine. Through her father she was the granddaughter of Adelaide of Turin and Odo of Savoy. While he was alive, her father was nominally lord of Savoy, Turin, and Susa, but real power remained in Adelaide’s hands. When Agnes was a teenager, her father died, leaving her heir to Turin and Susa. The County of Savoy had always been passed down in the male line, which meant Agnes had no claim on it, but the March of Turin had been inherited by daughters many times and there was uncertainty as to whether she or Amadeus had the right to it. Two years later, the question became obsolete with Amadeus’ death.
In order to ensure her inheritance though, Agnes had to do what her grandmother had done. She had to get married. She wed Frederick of Montbéliard, who was first cousin (through his mother) to Matilda of Tuscany. This had to have happened no later than 1080, when Frederick officially became Marquis of Turin. He didn’t take control though, since power remained firmly in Adelaide’s hands. Whether that power would eventually have passed to Agnes and Frederick is unknown. Frederick died five months before Adelaide did.
What happened to Agnes after that is somewhat unclear. She had three sons with Frederick, so presumably one of them took the title, but whether Agnes ruled as regent or simply alongside her sons as her grandmother had done is unknown. At some point, like so many other noblewomen, she retired to a monastery. The last known reference to her is the record of a donation she made to the monastery of Fruttuaria in 1110.
Agnes of Savoy - Epistolae
Previté-Orton. C.W. The Early History of the House of Savoy (1000-1233). Cambridge, 1912. - Archive.org