Lane-Claypon was born Janet Elizabeth Claypon in 1877 into an affluent family, in Boston, Lincolnshire, the daughter of william Ward Lane-Claypon, a banker and former first-class cricketer, and Edith (née Stow). Her uncle C. G. Lane was also a first-class cricketer. A few weeks after her birth, her father changed the family name to Lane-Claypon by royal licence.
In 1912, Lane-Claypon published a ground-breaking study of two cohorts (groups) of babies, fed cow's milk and breast milk respectively. Lane-Claypon found that those babies fed breast milk gained more weight, and she used statistical methods to show that the difference was unlikely to occur by fluke alone. She also investigated whether something other than the type of milk could account for the difference, an effect known as confounding.
In 1929, Lane-Claypon married civil servant Sir Edward Rodolph Forber (1878–1960), as his second wife. Forber held several prominent positions, including Deputy Secretary of the Ministry of Health. Lane-Claypon's final paper was published under her married name, and she essentially retired following her marriage, not uncommon for a woman of her class in this era.
Lane-Claypon tracked down 500 women with a history of breast cancer – the "cases" – and compared them with 500 women who were free of the disease but otherwise broadly similar, known as "controls".