Lally was educated at Hulme Grammar School in Oldham and worked as a secretary before walking-on and understudying at the Shakespeare Memorial Theatre in Stratford-upon-Avon. As a professional actress, she appeared in hundreds of stage productions, films and television programmes and rep at Manchester, Sheffield, Southport, Guilford, Liverpool, Birmingham and the Bristol Old Vic. Her London debut came in 1944 and her many West End successes included 'Dinner With the Family' (for which she won a Clarence Derwent award in 1957), 'Difference of Opinion', 'The Killing of Sister George', 'Dear Octopus' and 'The Beastly Beatitudes of Balthazar B'. Lally worked constantly until her death.