Mercedes Prat was born on 6 March 1880 in Barcelona in Spain. She was baptized on the following day, and made her First Holy Communion on 30 June 1890. From her childhood she gave herself completely to God, whom she received every day in Communion. She displayed a great love for her neighbor and tried to foster this kind of love in others. During her years in school, she was known for her goodness and her dedication to school work, excelling especially in painting and needlework, which were areas in which she had a natural talent.
Mercedes entered the novitiate of the Society of St Teresa of Jesus in 1904 in Tortosa, and became Sr Maria Mercedes. She was said to be a religious according to the heart of God: prudent and truthful, calm and gentle in her reactions, having a natural goodness in all her dealing with others, but firm in character. God was her one love, and her love for God kept growing to the point where she would give her life for Him. In 1920 she was assigned to the motherhouse in Barcelona. From there the path to martyrdom began on 19 July 1936, when her community was forced to give up the school and flee. On 23 July, for the crime of being a nun, Sr Maria Mercedes was arrested and shot; she died on the following day, in the early morning of July 24.