During World War I, Rohmer was a German MD officer at the military hospital of Metz. Some of his famous Prussian colleagues asked him to sign the Manifesto of the Ninety-Three in 1914, but his pro-French feelings made him refuse to sign it. After the Allies victory in 1918, he became the first Professor of Pediatrics at the French Medicine College of Strasbourg, which became part of France again after the war.
In 1920, Rohmer created the "Alsatian and Lorrainian Association of Nursery", the first one in France. The impacts of the work of this association were so huge that in 1945, more were created in France, on this model, the "National Mother and Childhood Protection" (also known as Protection maternelle et infantile ).
Rohmer was a pioneer in research into prematurity, poliomyelitis, tuberculosis, osteomalacia and vitamin C. He was the director of the pediatric clinic of Strasbourg until his retirement in 1947, and made it famous all around Europe. In 1946, he wrote with Robert Debré a famous manual entitled "Traité de Pathologie Infantile" (2,500 pages, two volumes) which became a reference for a whole generation of pediatricians. During his career, he was the physician of famous children, including those of the King of Belgium, of Konrad Adenauer, and of Pierre Pflimlin.
Despite his retirement at age 70 in 1947, Rohmer continued until age 100 to participate actively to his research dealing with childhood and teenagers. He died in Strasbourg in 1977. A street in the city is now called "Pr Paul Rohmer" to honor him.