After graduating in 1916 he worked briefly as a lecturer, but with the ongoing First World War he decided to leave the same year for France and joined the French Army as medical volunteer. He worked in the military hospital as a surgeon for the French Red Cross. With the Americanentry into the war in 1917, he transferred to the US Army and continued to work until 1919. In February that year, he was appointed a member of the Military Inter-Allied Commission to Germany; made chief of the Medical Service of the American Red Cross Commission to Germany and later made deputy commissioner of the American Red Cross in Europe. In this position he made studies of health conditions in Prague, Czechoslovakia and Kovno, Lithuania. After the war he ran a clinic in Manila and married Rosario Legarda Roces, whom he adopted a daughter with.
In 1922 he was asked to join the Philippine Constabulary and revitalize their medical services; he joined and had by 1926 been promoted to lieutenant colonel and chief surgeon, serving as medical inspector from 1926 to 1934. Valdes became brigadier general and chief of the Constabulary in 1934. He later took his oath of office as Deputy Chief of Staff of the Philippine Army on May 4, 1936, and with the retirement of Chief of Staff General Paulino Santos, Valdes assumed this office by presidential appointment on January 1, 1939. With the growing threat of Japanese expansion during the 1930s, President Manuel L. Quezon established the Department of National Defense in November, 1939, which had executive authority over the army. With the attack on Pearl Harbor and Japanese invasion of the Philippines in December, 1941, President Quezon merged the departments of National Defense, Public Works, Communications and Labor into a single department and appointed Valdes as secretary on 23 December. As a member of the War Cabinet, he was tasked by General Douglas MacArthur to be in charge of the safety of President Quezon, who was very ill by that time, and his family. They were all evacuated to Corregidor, then Australia, and finally to the United States, creating the Commonwealth government-in-exile. After the death of Quezon on August 1, 1944, Valdes continued to serve in President Sergio Osmeña's government with the same positions as before. When American troops invaded the occupied Philippines in the Second Philippine Campaign, Valdes returned together with MacArthur and President Osmeña in the landing on Red Beach, Leyte on October 20, 1944. Valdes reentered Manila on February 6, 1945, and was reunited with his family after three years of separation. Later the same month, the Commonwealth of the Philippines was reestablished and President Osmeña appointed Valdes as ad interimSecretary of Public Health and Welfare, officially taking the position on June 27, 1945. In this position he organized relief goods and medicine distributions from the U.S. Medical Corps to the war torn country. He retired from government service on July 4 the same year. Valdes, along with the future Secretary of Foreign AffairsRaul Manglapus, at the time a reporter for the Philippines Free Press, were the only two Filipinos accredited to join MacArthur during the signing of the Instrument of Surrender on board the USS Missouri in Tokyo Bay on 2 September 1945. Valdes received one of 20 original facsimiles of the Instrument of Surrender, being one of eight personal guests of MacArthur, and his document is currently owned and curated by The International Museum of World War II in Natick, Massachusetts. In January 1946 Valdes was appointed as one of the judges at the Military Tribunal of Japanese General Masaharu Homma in view of the war crimes committed by his command during the invasion of the Philippines, sitting on the bench along with Leo Donovan, Robert G. Gard, Arthur Trudeau, and Warren H. McNaught.
After the war Valdes went back to teaching as a professor of surgery at the University of Santo Tomas in Manila. He was head of the Philippine Cancer Society, vice-president of the Philippine Tuberculosis Society, chairman of the Deans Committee for the Veterans Memorial Medical Center and became the medical director of Our Lady of Lourdes Hospital from 1948 until his death. Basilio Valdes died on January 26, 1970, and was given a full military funeral.
