Marghanita was born in Manchester, England, to a prominent family of Jewish intellectuals (Neville Laski was her father, Moses Gaster her grandfather and Harold Laski her uncle), she was educated at Lady Barn House School and St Paul's Girls' School in Hammersmith, worked in fashion, then studied English at Somerville College, Oxford She married publisher John Howard (in Paris), and worked in journalism.
Marghanita Laski lived in Hampstead and Abbots Langley.
After her son and daughter were born, Laski began writing in earnest: Marghanita was an omnivorous reader, from 1958 onwards she became a prolific and compulsive contributor to the Oxford English Dictionary and by 1986 had "carded" around 250,000 quotations.
Laski was a panellist on the popular UK BBC panel show What's My Line? (that ran from 1951 to 1963), The Brains Trust (late 1950s), and Any Questions? (1960s).