Following in the family tradition, de Villegas de Saint-Pierre became a writer and published works under the pseudonyms "Quevedo" or "Dame Peluche". In 1902, she began publishing works and in 1912 published a novel,
Profils de gosses which was recognized by the
French Academy. She wrote a regular fashion column and covered galas and balls, but also wrote pieces of social commentary which were ahead of their time, including
L’influence féminine et les colonies (The female influence and the colonies),
La ligue nationale pour la protection de l’enfance noire au Congo belge (The National League for the protection of black children in the Belgian Congo) and
L’âge d’admission des enfants au travail (The entrance age of working children), publishing from 1913 in magazines like
La Femme belge. She was an associate of the
Society of Men of Letters. In 1907, de Villegas de Saint-Pierre became one of the founders of the Saint-Camille Nursing School (), a Catholic training facility, which educated nurses with a religious, rather than secular method. She believed that training was needed, but as a devout Catholic felt that science and religious training could co-exist. She undertook nursing classes herself, and upon hearing rumors of impending war, took steps to have the
Belgian Red Cross assist with training at the school. When
World War I began, she went to the Ministry of War to complete permits to have her castle at Chevetogne turned into a neutral hospital for troops on both sides of the conflict. The facility served as a temporary emergency station, called an "ambulance", and required modifications to be suitable, such as remaking the castle living room into an operating theater. At the end of August 1914, the Germans began occupying the castle and though de Villegas de Saint-Pierre and her nurses continued to work, she made plans to work at the front. Closing the hospital, in November, she traveled through the Dutch port of
Eijsden, staying with family until she could book passage on the ferry to
Calais. She was also able to briefly see her brother, Louis, who having been injured in battle, was serving as a consul in
Maastricht. Arriving in Calais, the Countess presented herself to Dr.
Antoine Depage, who refused her services for his famed . She then went to the Hospital du Duc de Vendome to offer her services and ended up working in the linen room of the hospital for six weeks. When her family doctor, Léopold Mélis, who was also the Chief Inspector of the Health Service for the Belgian army, discovered she was working as a seamstress and not a nurse, he transferred her to
Poperinge to help him establish a typhoid hospital there. The Élisabeth Hospital was established in the Castle d'Hondt with twenty-five English
Quaker nurses and some of her nurses from Saint-Camille. Much of her activity as director of the hospital was for fundraising and she used her wide contact with the nobility to garner money to both build and keep the hospital going. Initially, the focus was on the civilian population with a special focus on refugee and orphaned children, but in April 1915, when the Sacre-Coeur Hospital in
Ypres was closed, de Villegas de Saint-Pierre gained authorization to treat military personnel at Élisabeth Hospital. The Countess' vast circle of contacts and organization skills were utilized to found the Belgian Civil Help Association (), under the patronage of
Queen Elisabeth. This organization undertook supplying funds and medical necessities for the Sacre-Coeur Hospital in Ypres, Élisabeth Hospital and the Hospital of the Museum of Hazebrouck. It also provided funds and supplies for aid stations throughout the country, organized typhoid inoculations, distributed milk for children, helped with sanitary inspections and water purification programs, built two orphanages and established nurseries, as well as rescuing the cultural heritage and property of churches, convents and civic buildings. Through the Association, she organized schools in Poperinge in the garden of the castle, since most of the educational facilities had closed and teachers had fled the bombing. Over 300 children attended the daily classes, which were inspected by the Belgian authorities. Continued bombing forced removing the school to the Derycke Farm near
Zwijnaarde and then moving an additional 75 children to northern France where they attended a
Flemish school in
Caëstre. In 1916, de Villegas de Saint-Pierre became ill with a severe throat infection. A blood test revealed she had
sepsis and then she was overcome by
gas. She was invited by Clementine de Chaumont-Quitry, Baroness de La Grange, to convalesce at her castle near
Hazebrouck and a visit by Dr. Mélis, revealed there was a lump in her neck. He performed an immediate surgery, saving de Villegas de Saint-Pierre's life. After her recuperation, the Countess returned to her work at Élisabeth Hospital, until the war ended in 1918. She moved back to Brussels and retired from most of her hospital work, except serving on the boards of the Nursing Association, the Deux-Alice Clinic in
Uccle and the Saint-Camille Nursing School. Her war work was recognized by many honors including recognition as a Commander of the
Order of the Crown, a Knight of the
Order of Leopold, and as a Commander of the
Order of the British Empire, as well as receipt of the
French Croix de Guerre with palms and
Pro Ecclesia et Pontifice. After her husband's death, de Villegas de Saint-Pierre returned to writing, but using her own name Maria Van den Steen de Jehay. She published several autobiographical works, including
Mon journal d’infirmière (My Nursing Journal, 1923);
Nos souverains à La Panne (Our rulers in De Panne), which was serialized in the March and April 1939 editions of
Revue belge; and wrote two unpublished works
L’Autre Guerre (The Other War) and
British Area. ==Death and legacy==