Mechthild (German Mathilde) was born around 1125 at the Andechs Castle in Bavaria — daughter of Count Berthold I of Andechs and his wife Sophia of Istria. The House of Andechs-Meranien — to which Mechthild belonged — would produce the extensive 12th- and 13th-century Bavarian-European royal-saint dynasty that includes Sts. Hedwig of Silesia (1174–1243), Elizabeth of Hungary (1207–1231), and Agnes of Bohemia (1211–1282) — the kinswomen-saints of the High Medieval Catholic Central European royal-spiritual tradition.
Mechthild entered religious life at the Abbey of Diessen on Lake Ammer in Bavaria — one of the principal Bavarian women's-monastic foundations of the period, a house of Augustinian Canonesses Regular.
She became abbess of Diessen around 1153 — undertaking the reform of the Diessen community under the guidance of St. Hildegard of Bingen (the principal Rhineland visionary-abbess of the period, who maintained extensive correspondence with reforming abbesses and abbots across Catholic Europe).
Around 1156, at the request of Bishop Conrad of Augsburg, Mechthild was appointed abbess of the Edelstetten Abbey — a severely decayed Bavarian women's-monastic community requiring reform. Mechthild undertook the difficult reform under considerable opposition from the previous community, and died at Edelstetten on May 31, 1160 — during the hard reform period.
Her cult was immediately recognized at Diessen and at Edelstetten; the extensive Andechs-Meranien royal-saint dynasty — and especially the famous Sts. Hedwig and Elizabeth of Hungary — drew on the pre-existing veneration of the kinswoman-precursor Mechthild in the Bavarian-European royal-Catholic tradition.