Joseph Mason Baldwin, Australian astronomer, studied physics and mathematics at Queen's College, University of Melbourne, and was awarded the degrees Bachelor of Arts (1900), Bachelor of Science (1901), Master of Arts (1902) and Doctor of Science (1913). After 1902 he taught at Trinity College and Wesley College (University of Melbourne), and then at the Ballarat School of Mines. He also conducted research at the University of Melbourne for some time with Professor Thomas Lyle. On 18 July 1905 he arrived at the Royal Observatory, Cape of Good Hope, as a research student to complete a six month course in practical astronomy under the supervision of Dr David Gill*. He devoted himself mainly to a study of alpha Centauri, both visually and spectroscopically. He and R.T.A. Innes* used the Victoria telescope on eight days to make daylight observations of the position angles and distances of the components of alpha Centauri. Baldwin also assisted with other work using the heliometer and Victoria telescope, for example, he and other members of staff observed comet Finlay and comet Metcalf with the latter instrument. As a result J. Lunt* and Baldwin were able to publish "Observations of Comet 1906h (Metcalf)" in Astronomische Nachrichten (1907).
After leaving the Cape Colony Baldwin visited Potsdam, Paris, and observatories in England and the United States. He was appointed chief assistant to P. Baracchi at Melbourne Observatory in June 1908. During the next few years he published a number of papers on photometric measurements of Saturn (1908, 1909) and Neptune (1908), the short-period variable W. Ursae Majoris (1908), a star with a large proper motion (1911), and Comet Tuttle 1912b (1913). When Baracchi retired in 1915 he became acting director of the observatory and was appointed government astronomer of Victoria in 1920. Among others he directed the reduction of observations for the Melbourne zone of the International Astrographic Catalogue. He retired in 1943 and that same year Melbourne Observatory was closed.
Baldwin served on the council of the Royal Society of Victoria from 1920 to 1945, and as president in 1925/6. He was a large, kindly and soft-spoken person.