The Mainfest was originally a festival for fishermen and boatmen and was supposedly derived from the consecration of the Dreikönigskirche (Church of the Therre Kings) of 23 July 1340. The purpose of this festival was to give thanks and pay homage to "Their river and its bounties". They celebrated with "Roasted oxen on a spit", made wine flow from the Fountain of Justice (Gerechtigkeitsbrunnen) and enjoyed the entertainment. The attractions included passion plays staged by Schoolchildren in "huts" representing the "Kingdom of Heaven", goose plucking, duck catching by fishermen, gun salutes, wine fountains, illuminations, fireworks and colourful four- or six-in-hand carriage parades along the banks of the River Main.
The dedication of the new "Alte Brücke" (Old Bridge) brought back memories of old Mainfeste and the tradition of holding an annual public festival by the River Main was re-established. The destruction caused by the Second World War interrupted this tradition, however the festival was subsequently reinstated by Dr. Walter Kolb, the Lord mayor at the time. Construction work caused another break which saw the Mainfest move to the Ostpark for a short time, however it returned from its exile in 1973, making a sensational comeback to the Römerberg and the banks of the River Main.