Thom worked in the piece goods department of Jardine, Matheson & Co. where he acquired a knowledge of the Chinese language.
[1] When hostilities began between the British and the ruling Chinese Qing dynasty in late 1839, Thom, along with other Chinese translators including John Robert Morrison and Karl Gützlaff provided the necessary language interface between the warring factions.
In July 1840, during the First Opium War, Thom sailed north from Canton aboard HMS Blonde as translator to Captain Thomas Bourchier.
A number of Chinese from a mob gathered on the shore swam towards Thom's boat and he narrowly missed being hit by an arrow and musket fire as he shouted out the warning written on the placard.
Bourchier made good on his promise and shelled the fort and nearby warships before eventually withdrawing.