He is known for his numerous contributions in fluid mechanics covering a wide range of problem areas, such as flow instability and turbulence, gas kinetics, viscous compressible fluids and liquid helium flows.
He started working at Caltech in 1939 as a Research Fellow in Aeronautics. He became Assistant Professor of Aeronautics in 1945, Associate Professor in 1946 and Professor from 1949 to 1974. From 1974 to 1976 he was Professor of Aeronautics and Applied Physics, from 1976 to 1983 Charles Lee Powell Professor of Fluid Mechanics and Thermodynamics and 1984-85 von Kármán Professor of Aeronautics. Since 1985 he is von Kármán Professor Emeritus at Caltech. At Caltech Liepman has also been Director, Graduate Aeronautical Laboratories from 1972 to 1985 and Executive Officer for Aeronautics from 1976 to 1985.