Forssmann was educated at the university in his native city of Berlin where he qualified as a physician in 1929. He then worked in the 1930s as a surgeon in various German hospitals. After the war he practiced as a urologist at Bad Kreuznach from 1950 until 1958 when he moved to Düsseldorf as head of surgery at the Evangelical Hospital.

In 1929 Forssmann introduced the procedure of cardiac catheterization into medicine. He was struck by the danger inherent in the direct injection of drugs into the heart frequently demanded in an emergency. The alternative that he proposed sounded no less alarming – introducing a catheter through the venous system from a vein in the elbow directly into the right atrium of the heart. Drugs could then be introduced through this.

After practice on cadavers and an unsuccessful attempt on himself made with the aid of a nervous colleague, Forssmann decided to do the whole thing himself. He consequently introduced a 65-centimeter (25.6-in) catheter for its entire length, walked up several flights of stairs to the x-ray department and calmly confirmed that the tip of the catheter had in fact reached his heart. There had been no pain or discomfort.

Unfortunately further development was inhibited by criticism from the medical profession, which assumed the method must be dangerous. Consequently it was left to André Cournand and Dickinson Richards to develop the technique into a routine clinical tool in the 1940s; for this work they shared the Nobel Prize for physiology or medicine with Forssmann in 1956.