In 1957, Lagaly started his chemistry studies (as well as Physics, Mineralogy and Botany) at the Heidelberg University, receiving his degree as "Diplom-Chemiker" in 1962. He conducted his doctorate studies at the Institute for Inorganic Chemistry at the same institution under Armin Weiss, and was awarded his Doctor degree in 1967 with the work "Untersuchung von Quellungsvorgängen in n-Alkylammonium-Schichtsilicaten". From 1965 he worked as an Assistant at the Institute for Inorganic Chemistry of the Munich University where he concluded his Habilitation in 1971 and then was employed as lecturer ("Akademischer Rat/Oberrat"). In 1974 he was employed as Professor for Inorganic Chemistry at the University of Kiel, retiring in 2004.
His research lines concentrated on Clay chemistry, Colloid chemistry and Interface chemistry as well as chemistry from porous compounds, layered materials and intercalation chemistry.
Among his many contributions to the fields of clay and colloid chemistry, one of his most recognized developments was the Alkylammonium Method for the determination of layer charge in layered aluminosilicates.
He acted as treasurer of the German Colloid Society from 1979 to 2003. In 2004 the same institution awarded him with the Wolfgang Ostwald Prize. Lagaly was President of the German Clay Group (DTTG) from 1987 to 1992 and of the European Clay Groups Association (ECGA) from 1999 to 2003.
In addition, Lagaly acted for many years (1987-2004) as editor-in-chief for "Colloid and Polymer Science" and "Progress Colloid and Polymer Science" as well as editor for "Clay Minerals" (1982-1996) and "Applied Clay Science" (1985-1996).