Born in Linköping, Forseth was the son of the Norwegian lithographer Ole Andreas Forseth. He was raised in Örebro where his father ran a lithographic business. In 1905 the family moved to Gothenburg where he attended the Arts and Crafts School (Slöjdföreningens Skola) studying under Gunnar Hallström, Anders Trulsson and Charles Lindholm. He completed his education at the Royal Swedish Academy of Arts in Stockholm (1912-1915) under Olle Hjortzberg and Oscar Björck.
During his travels to Istanbul, Greece and Italy, he developed an interest in monumental and decorative art which extended to frescos, textiles and oil paintings. From 1921 to 1923 he decorated the Golden Hall in Stockholm's City Hall with mosaics in the Byzantine style, creating the Mälar Queen as the central figure.
Examples of his stained glass creations can be seen in St Mary's Church, Helsingborg (1937), Sankt Nicolai, Halmstad (1937), St Peter and St Sigfrid's Church, Stockholm, and Coventry Cathedral, for which he also created floor mosaics in 1962. He also contributed designs to Lidköping's porcelain factory.
- "Einar Forseth". Den Store Danske (in Danish). Retrieved 18 October 2014.
- "Einar Forseth 1892-1988" (in Swedish). Signaturer. Retrieved 18 October 2014.
- "Einar Forseth". Nationalencyklopedin (in Swedish). Retrieved 18 October 2014.
- "Einar Forseth" (in Norwegian). Store Norske Leksikon. Retrieved 18 October 2014.
- "Prins Eugen Medaljen" (PDF). Retrieved 14 February 2015.
- Böhn-Jullander, Ingrid; Forseth, Einar (1982). Einar Forseth: En bok om en konstnär och hans verk. Stockholms kommun.
- Stavenow, Åke Ludvig; Forseth, Einar; Odhe, Thorsten (1956). Einar Forseth: Swedish Painter. (Translated from the Swedish by Thorsten Odhe.) [With Reproductions and Portraits.]. Allhems Förlag.