After serving as Curate of the parishes of Lutterworth, Leicestershire (1851–1853), Escrick, Yorkshire (1853–1855), and Abbots Langley, Hertfordshire (1855–1859), he worked as a missionary in Moulmein, Burma (1859–1862), where he set up a school and translated part of the Book of Common Prayer into Burmese.
Between 1834 and 1848, Justus labors were confined to Moulmein district with occasional visits to Rangoon (now known as Yangon)and Tavoy (now known as Dawei).
Bassein and Moulmein must also have attracted some Panthay settlement, the latter port being a terminus of the overland caravan trade from Yunnan in its own right, via the northern Thai trade route through Kengtung, Chiang Mai and Mae Sariang.
At that time, due to political and religious persecution in Burma, predominantly Pwo-Karen from the hinterlands of Moulmein and Tavoy migrated into the area northeast of the Three Pagodas Pass, where they received formal settlement rights from the Siamese Governor of Kanchanaburi.