In 1947, Fides enrolled at the College of Music and Arts, where she studied with Dean Felicing Tirona. Her graduation recital landed her on the front page of the Manila Times, which proclaimed "A star is born". The programme included Zerbinetta's Aria from Strauss' Ariadne auf Naxos, an aria which, to this date, has never been sung by another artist in the Philippines. After graduation, she received a scholarship to study at the Curtis Institute of Music in Philadelphia (the first Filipina singer ever admitted). At the Institute, she performed The Telephone and the composer, Gian Carlo Menotti, commented "It's as if I composed this work for her."
After her marriage in 1954 to Manuel D. Asensio, Jr., she returned to Manila (where she would be the reigning coloratura soprano for three decades) and made her professional opera debut as Adele in Die Fledermaus at the Far Eastern University Auditorium. Two years later, she sang the role of Sisa in the world premiere of De Leon's Noli Me Tangere with the Manila Symphony.
Until the late 1960's, Fides Asensio appeared in numerous Filipino operas, including the world premieres of Santos' Mapulang Bituin, Pajaros' Binhi ng Kalayaan, and Kasilag's Dularawan. Her repertoire also included roles from conventional opera, including Gretel in Hänsel und Gretel, Lucia in Lucia di Lammermoor, Susanna in Il Segreto di Susanna, and Konstanze in Die Entführung aus dem Serail.
From 1969-1974, Asensio hosted "Sunday, Sweet Sunday", a one-hour show where she performed in major musical works. It was also around this time that she added two new roles to her repertoire: Violetta in La Traviata and Susanna in Le Nozze di Figaro.
In the late 1970's, Asensio began to direct operas for free in Manila since there were very few directors at the time who were interested in opera. During this time, she also wrote the librettos for Kasilag's Larawan Ng Kababaihan: Maskara at Mukha (debuted in 1980) and Feliciano's La Loba Negra (debuted in 1984). Both of these works were staged to great popular and critical acclaim at the Cultural Center of the Philippines in Manila.
In 1986, Fides Asensio formed "The Music Theater Foundation of the Philippines" (MTFP), a non-profit organization dedicated to promote, stage, and give scholarships to young classical performers. Two years later, in 1988, she was appointed Chairman of Voice and Music Theater Department at the U.P. College of Music, a position she held until she retired in 1997. After her retirement, she was granted the title of Professor Emeritus by the U.P. Board of Regents. She continues to teach, and among her students are tenor Salmyeno Malaki (who performs in Los Angeles and San Diego), soprano Andion Fernandez (currently under contract with the Deutsche Staatsoper), and tenor Lemuel Cuento (currently under contract with the Vienna Staatsoper).
In 1989, Asensio began hosting a second television series, "A Little Night of Music." The show is still being produced and she continues to host and perform.
In 1992, Asensio began working on the libretto for Paguio's Mayo... Bisperas ng Liwanag for the Philippine Centennial. The opera, which is based on Nick Joaquin's short story "May Day Eve", had its world premiere at the Camp Aguinaldo Theater in Manila on November 15, 1997. The premiere was a great success and Asensio translated the libretto into English for staging in Aberdeen, Scotland, in April 2000.
On April 15, 1999, Asensio created the role of Grandmother Sakuro in the world premiere of Tamura's The Magic Staff. The performance, which was staged at the Meralco Theater in Manila, may have made history when Fides appeared with her granddaughter, Nicole, who was also appearing in the production.
Although her career has focused mostly on classical theater, Asensio once ventured into film when she appeared (and was nominated for best supporting actress) in the epic "Oro, Plata, Mata."