Mallam was born in Kensington, London, and in 1902 emigrated to Adelaide, South Australia, where his uncle George Bessant Mallam (1843–1910) was a successful medical practitioner who had in 1884 married Annie Isabel Kyffin Thomas (1864–1948), daughter of the influential newspaper proprietor William Kyffin Thomas (1821–1878).
He began practising law in Kadina in 1903, then a year later in Adelaide, where he also served as managing clerk for Paris Nesbit.
[3] There he was a popular barrister and often took on 'the establishment' in court and he strongly opposed the administration of John A. Gilruth, whom he believed to be inept.
[8]In November 1928 he sat on the trial of Padygar and Arkikra, two Aboriginal men who had been charged with the murder of Frederick Brooks at Coniston Station.
[4] He retired in 1933 due to ill health and his vacant position on the Supreme Court was filled by Thomas Alexander Wells.