Born to English emigrant parents in New Zealand, Joan Hammond's family moved to Sydney, Australia, soon after her birth and in school there began to study singing as well as violin and piano. She was also an excellent athlete and won the New South Wales golf championship several times. At the beginning, her primary musical interest lay in the violin, but an injury to her left arm forced her to concentrate on her singing. In 1932, Hammond was among the young artists chosen to sing with the Williamson Imperial Grand Opera Company which brought such great artists as Pagliughi, Castagna, and Granforte to Australia. She was given several smaller roles along with work in the chorus. In 1936, she left Australia for Vienna and then to London to further her studies. In London, she worked with Italian tenor Dino Borgioli. She returned to Vienna to make her professional debut as Nedda in I pagliacci at the Volksoper. In 1938 she returned to London to sing Handel's Messiah under the direction of Thomas Beecham and then returned to Vienna to continue singing at the Volksoper and the Staatsoper. Her roles included Mimi, Violetta, Martha, and Constanze. She then moved to Aussig where her contract called for her to sing Micaëla, Mimi, and Mignon and the following year she sang Elisabeth in Tannhäuser and Donna Elvira in Don Giovanni at the Vienna Staatsoper.
She returned to London in August 1939 for a Proms concert; because of the outbreak of war she was stranded there with only the belongings she brought with her for the concert. During the war she sang many concerts and recitals for the troops and drove an ambulance during emergencies. At this time she joined the Carl Rosa Opera Company to sing in Faust, La traviata, La bohème, Tosca, and Madama Butterfly. After the war she was the first British singer to appear at the Vienna Staatsoper. She returned to London in 1948 for her Covent Garden debut in Il trovatore and the following season sang Butterfly, Tosca, and Aida at the New York City Opera. She continued to sing with many of the major opera companies around the world, and in 1957 she was the first British singer to sing in Russian on a Russian stage when she performed Eugene Onegin. Her career was interrupted in the 1960s by a heart ailment which forced her retirement in 1965. At that time she returned to Australia and in 1974 she was made a Dame of the British Empire. Her autobiography, A Voice, a Life, was published in 1970.
Hammond's voice had a bright silvery quality. Her many recordings display an excellent command of phrasing and an ease in the higher register. Her most famous recording is "O mio babbino caro" from Gianni Schicchi — an aria rarely recorded until Hammond made it one of the most requested arias in opera.