After receiving a bachelor's degree in education from Bloomsburg State Normal School in 1894, he entered the Medico-Chirurgical College of Philadelphia, from which he graduated with the M. D. degree in 1898.
Pfahler did his internship (1898 - 1899) and residency (1899 - 1902) at the Philadelphia General Hospital. He was then appointed clinical professor of symptomatology at the Medico-Chirurgical College, and served in that position until 1908.
In 1909 he became the first clinical professor of radiology at the college, and was made a full professor two years later. The Medico-Chirurgical College and its faculty joined the University of Pennsylvania Graduate School of Medicine in 1916. Pfahler was appointed vice-dean for the department of radiology at the university in 1933. During these years he also maintained a private practice in Philadelphia, and served as a consultant in radiology at Misericordia, Hahnemann, General, and Woman's Medical College hospitals in that city.
Pfahler began his work with X rays just four years after Wilhelm Roentgen announced their discovery in 1895, and just one year after the Curies made their discoveries about radium. His work was with both the diagnostic and the therapeutic uses of the rays. Pfahler was president of the American Roentgen Ray Society in 1910, and the first president of the American College of Radiology in 1922-1923. He was also president of the American Electrotherapeutic Association (1912) and of the American Radium Society (1921 - 1922). He was a diplomat of the American Board of Radiology and a fellow of the American College of Physicians. He also devoted time to Philadelphia-area dermatological, cancer, and aid societies.
Ironically, his researched of X-ray techniques caused his disfigurement, poor health, and death. He died at Philadelphia, of leukemia caused by exposure to excessive amounts of radiation.
Pfahler married Frances Simpson on November 8, 1908. She died two years later, and on July 10, 1918, Pfahler married Muriel Bennett. There were no children from either marriage.