The first plans for a defensive line on the Karelian Isthmus were ordered by Mannerheim from Swedish volunteer Lt.Col. A. Rappe at the beginning of May 1918. Rappe's line was placed close to the border and designed to protect two rail lines which crossed the border, which would be used in a counterattack to Leningrad. When Mannerheim resigned at the end of May, Rappe's plans were abandoned.
Meanwhile Germans had ordered Col. O. von Brandenstein to investigate defensive positions at the Karelian Isthmus, and he delivered his plan July 16. He was the first to suggest using the Karelian Isthmus, Lake Suvanto and the Vuoksi river as defensive positions, and his plan was initially approved by Finnish high command in August 1918. In October 1918 the Finnish government allocated 300,000 marks for the work, which was to be carried out by German and Finnish sappers as well as Russian prisoners of war. However, the money allocated was insufficient and a lack of building materials and qualified workforce prevented the building of proper fortifications. When Germany lost World War I, von Brandenstein's plan was scrapped.
During October 1919 Finnish Chief of Staff Maj. Gen. O. Enckell decided the place of the line, mostly following the original location von Brandenstein had presented. Major J. Gros-Coissy, a member of the French military commission, designed the fortifications together with Finnish Lt.Col. J. Fabritius.
The line was constructed in two phases: 1920-1924 and 1932-1939. During the later phase some older fortifications were modernized.
The line ran from the coast of the Gulf of Finland through Summa to the Vuoksi River and ended at Taipale. It consisted of 157 machine gun positions and eight artillery positions made from concrete. The area around Summa was the most heavily fortified because it was thought to be the most vulnerable position. The first bunkers were built between 1921 and 1924. A second phase began in 1932, but was interrupted by the Winter War.
On the coast of the Gulf of Finland the line was guarded by Fort Saarenpää and on the coast of Lake Ladoga (Laatokka) by Fort Järisevä. These coastal artillery positions had 5", 6" and 10" cannon.
Unlike the French Maginot Line and other similar forts made with huge bunkers and lines of dragon's teeth, the Mannerheim Line was mostly built by utilizing the natural terrain. Many natural things such as fallen trees and huge boulders were used as defensive positions. The Finns also mastered camouflage techniques, which they put to use when building this defensive line.
The name Mannerheim Line was supposedly coined by Jorma Gallen-Kallela, and spread by foreign journalists.
During the war both Finnish and Soviet propaganda considerably exaggerated the extent of the line's fortifications: the former to improve national morale, and the latter to explain their troops' slow progress against Finnish defenses. Consequently, the myth of the "heavily fortified" Mannerheim Line entered official Soviet war history and also some Western sources. However in reality, the vast majority of the Mannerheim Line was comprised merely of trenches and other field fortifications. Bunkers along the line were mostly small and thinly spread out, and the Line had hardly any artillery.