[5] Malmkwist is notable for having recorded in more than 10 languages, including Swedish, English, Finnish, Norwegian, Danish, Dutch, French, Italian, Spanish and particularly German.

[9] She won the first round of competition locally in Skåne, and got to travel to Stockholm Concert Hall where the grand final took place.

Her next song, "Läs inte brevet jag skrev dej", sold over 80,000 copies.

[20] In 1964, she won the competition with "Liebeskummer lohnt sich nicht", which sold 1,000,000 copies and peaked at number one in the West Germany chart for ten weeks, In Swedish the song was named "Kärleksgrubbel".

[24][25] She soon also showed to have talent on stage; she had the leading role in the musical Irma la douce.

[26] She also participated in the Kar de Mumma-revue at Folkan in 1972, and in the Karl Gerhard show Hej på dig du gamla primadonna at Vasateatern.

[26] She received acclaim for her role as Luise in the musical Nine at Oscarsteatern 1983–84, where she acted against Ernst-Hugo Järegård.

[27] In 2008, as Fräulein Schneide in Colin Nutley's version of the musical Cabaret at Stockholms stadsteater.

[29] She participated in Melodifestivalen 2004 with the song "C'est la vie" along with Towa Carson and Ann-Louise Hanson.

[44] Siw Malmkvist was married to Finnish-Swedish musician Lasse Mårtenson between 1963 and 1969, and the couple have one daughter.

[45] Malmkvist was in a relationship with actor Fredrik Ohlsson from 1971 until his death in 2023; the couple have a son together born in 1973.