According to the Cornish lists, Marwenna was one of the many holy daughters of King Brychan Brycheiniog, but she is quite distinct from her similarly named sister, St. Morwenna. She also not be confused with the 10th century St. Merewenna, Abbess of Romsey in Hampshire.
St. Marwenna grew up at her father's Royal Court in Talgarth, before travelling across the Bristol Channel to Cornwall. Here, she settled at Marhamchurch, near Bude, on the site of the present church. She lived their in holy seclusion until her death on 12th August some time in the mid-6th century.