Alt received his Abitur in 1965 from the prestigious secondary school Helmholtz-Gymnasium Hilden. In 1971 he received his PhD from the Georg-August-Universität Göttingen under Erhard Heinz with thesis Verzweigungspunkte von H-Flächen (Branching points of H-surfaces.)
Alt became a professor at the Institute for Applied Mathematics at the University of Bonn, where he retired as professor emeritus in 2010. In 2011 he was made an honorary professor at the Technical University of Munich.
with Stephan Luckhaus: Wilhelm Alt, Hans; Luckhaus, Stephan (1983). "Quasilinear elliptic-parabolic differential equations". Mathematische Zeitschrift. 183 (3): 311–341. doi:10.1007/BF01176474. S2CID120607569.