In 1926 Krebs joined Otto Heinrich Warburg as a research assistant at the Kaiser Wilhelm Institute of Biology in Berlin. In 1932 he worked out the basic chemical reactions which established his scientific reputation. Krebs was terminated from his job on July 1, 1933. Sir Frederick Gowland Hopkins at the university of Cambridge persuaded the university to recruit Hans to work with him in the Department of Biochemistry. University of Sheffield opened a Department of Biochemistry in 1938 and Krebs became its first Head, and eventually professor in 1945. Hans settled and worked in Cambridge.

At the University of Sheffield, Krebs and William Johnson investigated cellular respiration by which oxygen was consumed to produce energy from the breakdown of glucose. in 1937, biochemists Franz Koop and Carl Martinus had demonstrated a series of reactions using citrate that produced oxalocetate. After four months of experimental works to fill the gap, Krebs and Johnson succeeded in establishing the sequence of the chemical cycle, which they called the “citric acid cycle.” The Citric Acid Cycle, or the Krebs Cycle, is a series of chemical reactions used by all aerobic organisms to generate energy through the oxidation of acetyl-CoA derived from carbohydrates, fats and proteins into carbon dioxide and chemical energy in the form of GTP.