Jahn received a Master of Science degree from John Casimir University (nowadays Ivan Franko National University of Lviv) in 1937. Two years later he was given a Doctor of Philosophy degree from the same university.
Jahn survived the Nazi occupation of Poland by working as a feeder of lice at Rudolf Weigl's typhus research institute in Lviv. Then he worked as a docent at the Maria Curie-Skłodowska University. From 1949 Alfred was a professor at the University of Wrocław. Also Alfred served as a director of the Institute of Physical Geography/Geomorphology at the same university from 1958 to 1968. In 1962, he was appointed a president of the University of Wrocław and had held it for six years. In 1985, Jahn retired from that university.
In the 1950s, Alfred participated in expeditions to Spitsbergen and the Polish Polar Station in Hornsund. He also conducted research in Siberia, Alaska and other parts of Scandinavia which made him one of the foremost polar geomorphologists in the world. In addition, Jahn published various works on the Polish part of the Sudetes Mountains emphasizing the role of climate in shaping the mountains.
Jahn was a member of the Polish Academy of Science, Norwegian Academy of Science and Letters and German Academy of Sciences Leopoldina.
In 1972, he founded the Polar Club of the Geographical Society of Poland and served as its first president until 1982. Alfred was a chairman of the committee on Polar Research of the Polish Academy of Sciences from 1981 to 1984.
On August 3, 1940 Alfred Jahn married Maria Szaynowska. They have two children.