She was born Jeanne Paule Deckers in Laeken in 1933, the daughter of a pâtisserie shop owner, and was educated in a Catholic school in Brussels. She was a keen Girl Guide who bought her first guitar to play at Guide evening events. Though she was thinking about becoming a nun even as a young woman, she trained and then worked as a Teacher.
She moved in with Annie Pécher (1944-1985), whom she had first met when she worked as a counsellor in a seaside camp in her youth. Annie, who was 11 years her junior, became warmly attached to her, a sentiment that Deckers did not reciprocate at the time. Nevertheless, Pécher visited Deckers regularly in her convent, went to live near where Deckers stayed when sent to study at Leuven, and even fell into a deep depression and tried to kill herself when it seemed Deckers was about to be sent to a mission country.
In September 1959 she entered the Missionary Dominican Sisters of Our Lady of Fichermont, headquartered in the city of Waterloo, where she took the name Sister Luc-Gabrielle.
In 1963 she was sent by her order to take theology courses at the University of Louvain. She liked the student life, if not her courses. She reconnected with a friend from her youth, Annie Pécher, with whom she slowly developed a very close relationship.
In 1961, the album was recorded in Brussels at Philips; the single "Dominique" became an international hit, and in 1962 her album sold nearly two million copies. The Dominican Sister became an international Celebrity, with the stage name of Sœur Sourire ("Sister Smile"). She gave concerts and appeared on The Ed Sullivan Show on 5 January 1964. "Dominique" was the first, and remains the only, Belgian song to be a number one hit single in the United States.
In 1965, Debbie Reynolds starred in The Singing Nun, a biographical film loosely based on Deckers. Deckers reportedly rejected the film as "fiction".
The Singing Nun is a 1966 American semi-biographical film about the life of Deckers. It stars Debbie Reynolds in the title role. The film also stars Ricardo Montalbán, Agnes Moorehead, Katharine Ross, Chad Everett, and Ed Sullivan as himself.
Increasingly frustrated at what she perceived to be the Catholic's Church failure to fully implement the reforms of the Second Vatican Council she released a song in 1967 defending the use of contraception called "Glory be to God for the Golden Pill" This led to an intervention by the Catholic hierarchy in Montreal, Quebec, Canada, which resulted in one of her concerts being cancelled.
In 1973, Deckers became involved with the Catholic Charismatic Renewal. Cardinal Suenens requested that she write songs for the movement, and this led to a brief but successful return to the stage, including a visit to Pittsburgh, where she sang before several thousand people. Under the name "Sister Smile", she released another album in 1979, which she described as containing "honest, religious songs" and commented that the album would help listeners to "know who I really am."In the late 1970s, the Belgian government claimed that she owed $63,000 in back taxes. Deckers countered that the royalties from her recording were given to the convent and therefore she was not liable for payment of any personal income tax. She then called on her former convent and her former production house, Philips. If the sisters gave her what they considered to be her share (helping her to acquire her apartment in Wavre, on condition that she stopped denigrating the congregation and signed a document for the balance of all accounts), Philips, which had received 95% of the revenue, does nothing. Deckers ran into heavy financial problems. In 1982, she tried, once again as Sœur Sourire, to score a hit with a disco synthesizer version of "Dominique", but this last attempt to resume her singing career failed. In addition to the other financial worries, an autism centre for children started by Annie Pécher had to close its doors for financial reasons in 1982. After this Deckers tried to make a living by giving lessons in music and religion.
Deckers found it difficult having to live up to her publicity as "a true girl scout", always happy and in a good mood. "I was never allowed to be depressed", Deckers remembered in 1979. "The mother superior used to censor my songs and take out any verses I wrote when I was feeling sad."
Citing their financial difficulties in a note, she and Annie Pécher committed suicide by taking an overdose of barbiturates and alcohol on 29 March 1985. In their suicide note, Deckers and Pécher stated they had not given up their faith and wished to be buried together after a church funeral. They were buried together on 4 April 1985 in Cheremont Cemetery in Wavre, Walloon Brabant, the town where they died. The inscription on their tombstone reads "J'ai vu voler son âme/ A travers les nuages" (English: "I saw her soul fly through the clouds").
In 1996, The Tragic and Horrible Life of the Singing Nun premiered Off-Broadway at the Grove Street Playhouse. The play, which was written and directed by Blair Fell, was loosely based on the events in Deckers' life. The production featured several musical numbers and followed the renamed character Jeanine Fou's life from her entry into the convent until her death with Pécher. The New York Times review stated the play "milks much of its comic mileage from the incongruous, and willfully tasteless, pairing of its holy setting and its trashy, Jacqueline Susann-style dialogue ... In dressing up despair in barbed frivolity, Mr. Fell provides his own skewed equivalent of tragic catharsis." The Catholic League spoke out publicly against the production.
In 2006, a musical version of Fell's play was staged during the New York Musical Theater Festival, produced by George Demarco and David Gerard, both of whom produced the 1996 production. Laura Daniel played Jeanine and received the NYMF Award for Outstanding Individual Performance. The musical featured music and lyrics by Andy Monroe and a book by Fell (who also contributed additional lyrics); it was directed by Michael Schiralli.
In 2009, Sœur Sourire, a Franco-Belgian biopic, starring Cécile de France as Deckers, was released.