As an English name it has been very popular since the Middle Ages. It was the top name for girls in England and Wales in the 1920s, 30s and 40s, but it declined in the latter half of the 20th century.
Other saints by this name include a queen of Scotland and a princess of Hungary. It was also borne by Queen Margaret I of Denmark, who united Denmark, Sweden, and Norway in the 14th century. Famous literary bearers include American writer Margaret Mitchell (1900-1949), the author of Gone with the Wind, and Canadian writer Margaret Atwood (1939-). Others include American anthropologist Margaret Mead (1901-1978) and British prime minister Margaret Thatcher (1925-2013).
As a Christian name, Ruth has been in use since the Protestant Reformation. In England it was associated with the archaic word ruth meaning "pity, compassion" (now only commonly seen in the word ruthless). The name became very popular in America following the birth of "Baby" Ruth Cleveland (1891-1904), the daughter of President Grover Cleveland.