According to the Europäische Stammtafeln genealogy, he was probably the elder son and heir of Count Arnold of Dießen (d 1098) and his wife Gisela of Schweinfurt, a daughter of Duke Otto III of Swabia. He thereby was a grandson of Count Frederick II of Dießen (d 1075). In 1098, he inherited his father"s Bavarian possessions around the Ammersee and Lake Starnberg, as well as those in Upper Franconia.
He was a ruling count of Dießen and Andechs in the Duchy of Bavaria, of Plassenburg and Kulmbach in Franconia, as well as bailiff of Benediktbeuern Abbey. Berthold"s descendance has not been conclusively established. Around 1120, he could succeed the comital Sigimar dynasty as bailiff (Vogt) of Benediktbeuern Abbey, and thereby increased his influence considerably.
He co-founded the abbey of Dießen (where in 1130 he was documented to be bailiff) and maintained relations with Admont Abbey in the March of Styria.
Around 1100, Berthold built a new residence on a hill outside Andechs, which later became the site of Andechs Abbey. A little later, in the early 1130s, he built Plassenburg Castle north of Bayreuth.
From 1137, he styled himself "Count of Plassenburg". He probably founded the town of Kulmbach around the same time at the foot of the hill below Plassenburg Castle.
The town is first mentioned in contemporary sources in 1174.
She brought more prestige into the marriage, and the family"s first possessions southeast of the Alps. Between 1143 and 1147, Bethold became Count of Stein. After 1140, Berthold spent much of his time at the court of King Conrad III of Germany.
This was probably related to a dispute he had with the Archbishop of Bamberg, who felt that Berthold threatened his worldly power in Upper Franconia.
