Born in Mainz, Johannes Krahn studied architecture from 1923 to 1927 at the Technische Lehranstalten Offenbach. He continued his studies 1927 to 1928 at the Kölner Werkschulen as Meisterschüler of Dominikus Böhm, who interested him in building churches. Krahn worked with Rudolf Schwarz from 1928 to 1940. He graduated as a civil engineer at the RWTH Aachen University.
In Frankfurt he was in charge of the rebuilding after World War II of the Paulskirche, starting in 1947, later he was on the team to rebuild the Städel. In 1950 he built the French Embassy in Bad Godesberg. In 1954 he completed at the Konstablerwache in Frankfurt the early skyscraper Bienenkorbhaus (Beehive House).
His church building St. Wendel, Frankfurt (1957) has been compared to Le Corbusier in terms of materials and flow of light. In 1962 he built the Klaus-von-der-Flüe Friedenskirche in Wörsdorf In 1965 Krahn created a variation on the interplay of stonemasonry, glass and concrete of St. Wendel in St. Martin, Idstein. In 1966 he built St. Sebastian in Frankfurt. In 1973 he built the City-Hochhaus in Frankfurt, together with Richard Heil.
Among his last projects was in 1974 the design of St. Aegidius in Bonn-Buschdorf, realized after his death by his son Johannes Krahn in the firm Krahn-Lorenz-Sauer, Frankfurt am Main, in 1978 to 1980. He died in Orselina, Switzerland.
Krahn was a teacher at the Kunstgewerbeschule Aachen and since 1954 a professor of architecture at the Städelschule Frankfurt. He was the director of the Städelschule from 1965 to 1970. With dedication to detail, he stressed function, construction and material, and refrained from pure ornament.
St. Wendel Chruch is situated in Sachsenhausen, a neighborhood in the south of Frankfurt am Main. The longitudinal hall church has a round apse on its eastern end. It is 42 meters long, 15 meters wide and 16 meters high.
On its north-west corner there is a 34 meters high round bell tower. On the north-east corner there is a small weekday chapel repeating the the floor-plan of the church in a smaller version.
The church walls are made of undressed trachyte stones. The load bearing concrete elements are left visible. Sixteen columns carry the wall panels which are structured by horizontal and vertical strip windows.
The strip windows covered ornamental wired glass are placed on floor level and just under the ceiling, creating the impression of the heavy quarry stone walls floating above the floor. A wide full height ornamental window is situated in the southern facade, just next to the altar.