Lucia was born on March 22, 1907, in Aljustrel, in the parish of Fatima. She was the youngest of seven children born of Antonio dos Santos and Maria Rosa.

Lucia's life changed greatly during and after the appearance of Our Lady.  She was ridiculed and scoffed at and imprisoned.  She never lost her devotion.  The apparition site was being trampled by pilgrims thereby making it impossible for the land to be nourished and Lucia was blamed for this. She was beaten by her mother with a broom stick or stick of wood but Lucia felt "she was right to think me unworthy of such a favor".

In 1921 the Bishop told Lucia to attend the school of the Sisters of St. Dorothy near Oporto where she would be unknown. At the school the Mother Superior changed her name to Maria of the Sorrows. Lucia was told not to speak about the apparitions. In 1925 Lucia joined the Sisters of St. Dorothy.

On December 10, 1925, Our Lady, with the Child Jesus beside Her, appeared to Sister Lucia in her room. She was holding out Her Heart surrounded by sharp thorns. Both asked for reparation to be made to the Immaculate Heart. Our Lady promised to assist with all the necessary graces for salvation at death of those who would make the Five First Saturdays. That is, those who, for five consecutive first Saturdays of the month, would go to Confession, receive Holy Communion, recite the Rosary and spend fifteen minutes meditating on the Mysteries of the Rosary, all with the intention of making reparation to Her. (Later Our Lord said that Confession could be on another day as long as one receives Holy Communion in the state of grace.)

On May 29, 1930 Our Lord explained to Sr. Lucia that the five Saturdays were in reparation for:

1. Those who deny the Immaculate Conception.
2. Those who deny Our Lady's virginity.
3. Those who deny Our Lady's Divine maternity and that She is the Mother of all
mankind.
4. Those who cause children to have indifference, contempt or hatred toward Our Lady.
5. Those who commit outrages against Holy Images of Our Lady.

In 1927, at the Convent in Spain to which she was transferred, Lucia, in a vision,
received permission to reveal two parts of a three-part secret which she received from Our Lady at the Cova da Iria during the July 13th apparitions: the vision of Hell and the urgent need for devotion to the Immaculate Heart. The third part of this secret was given to the Bishop and later was given to the Pope. Sr. Lucia asked that it not be made public before 1960. This part of the secret has never been made public.

In 1929, during Lucia's weekly Thursday Holy Hour between 11 P.M. and midnight,
she saw a cross of light appear above the altar. On the cross was Jesus Crucified. From Our Lord's wounded side and His face drops of Blood fell onto a Host, then into a chalice below, which were suspended in the air. From the left arm of the cross large letters reading "Grace and Mercy" made of what appeared to be water ran down upon the altar. On the top of the cross was the image of the Eternal Father. Before Him was a dove of light representing the Holy Spirit. Beneath the right arm of the cross was Our Lady of Fatima with Her Immaculate Heart in Her left hand. In the midst of this vision Our Lady made a request for the Pope to consecrate the world to Her Immaculate Heart in union with all the Bishops of the world, making a special mention of Russia.

This request was brought to the attention of the Pope.

 In 1938 unusual lights appeared in the sky which, according to Lucia, was the sign promised by Our Lady before God's punishment of the world. Shortly thereafter, WWII began during the pontificate of Pope Pius XI. In 1942, Pope Pius XII consecrated the world to the Immaculate Heart. He did not, however, do this in union with all the Bishops. Our Lord told Lucia that He was pleased with this act although it wasn't all that He asked for. He promised to end World War II. But the conversion of Russia would not take place.
 

In 1948 Sr. Lucia entered the Carmelite convent in Coimbra, Portugal.
Her name is now Sr. Maria of the Immaculate Heart of Mary.

An attempt was made on the life of Pope John Paul II on May 13,1981. He credited Our Lady of Fatima with saving his life. He made a pilgrimage to Fatima the following year.  On March 25, 1984, Pope John Paul II consecrated the world to the Immaculate Heart of Mary. According to Sr. Maria Lucia this consecration was accepted by Our Lady.

Sister Maria Lucia of the Immaculate Heart is still living. She wrote her memoirs and they have been  published as  "Fatima, in Lucia's Own Words".