She was born Isobel Morag Marshall in Aberdeen, Scotland, the daughter of a doctor. She went to the independent Mount School in York and, following in her father's footsteps, studied medicine at Glasgow University. She qualified as a doctor in 1940, and married solicitor and company director Geoffrey Barnett the following year. He was knighted for political and public services to the city of Leicester in 1953. Lady Barnett gave up her medical career in 1948 and for the next twenty years was a Justice of the Peace.
Radio and television
In 1953 she arrived on BBC television as one of the panel of What's My Line, which made her a household name. She appeared on the programme for ten years. She was not an original panelist, her seat having been previously occupied by Marghanita Laski.
Elegant and witty, she was regarded by audiences as the epitome of the British aristocracy (although her title actually came from the fact that her solicitor husband had been knighted; the incorrect form Lady Isobel Barnett suggested she possessed a courtesy title, but she was not an aristocrat, nor had she married into the aristocracy). She also made regular appearances on the BBC radio series Any Questions, on the radio panel game Many a Slip and on the women's discussion series The Petticoat Line. The crystal-clear voice and discreet and engaging smile also made Lady Barnett greatly in demand as an after-dinner speaker, a role into which she slipped confidently, always delivering a highly amusing and perfectly polished speech. In 1956, a reviewer predicted that an alien visiting from another planet could ask anyone between the ages of seven and 70 "What is What's my Line?" and "Who is Isobel Barnett?" and be confident of getting an answer. She featured in the first revival of What's my Line? which ran for 2 series in 1973 and 1974.
Later life and death
In her last years Barnett became reclusive and eccentric. In 1980, she was found guilty of shoplifting, being fined £75 for stealing a can of tuna and a carton of cream worth 87p from her village grocer. This brought her briefly back into the public eye; just four days later, she was found dead, having electrocuted herself in the bath.
Her story was sensitively recounted by several of her friends and colleagues in a 1991 BBC Radio 4 documentary in the Radio Lives series, which confirmed that she gave no indication whatsoever to any of her friends that she was planning to take her own life, and that she kept up a façade of "business as usual".
She had one son, Alastair (born 1944). Her husband Sir Geoffrey died in 1970.
Lady Barnett's autobiography, My Life Line, was published in 1956.