He won his Lieutenancy by ‘repeated acts of conspicuous gallantry’, as Mate of the Calliope in the China War (1841),[3] assisted at the destruction of a 20-gun battery at the back of the island of Anunghoy on 23 February 1841[5] and on 13 March 1841 served in the boats at the capture of several rafts and of the last fort protecting the approaches to Canton.

[6] He was similarly employed at the capture of that city on 18 March 1841[7] and, during the second series of hostilities against it, was afresh engaged in the boats at the destruction on 26 May of the whole line of defences, extending about two miles from the British factory.

Fitzjames recommended Le Vesconte's appointment to the discovery-ship Erebus[10] under Captain Sir John Franklin, which he joined on 4 March 1845 as she was fitting out for the polar expedition at Woolwich Dockyard[3] and in which he was involved in a renewed attempt to explore the Northwest Passage through Lancaster Sound and Bering Strait.

[11] At the Whalefish Islands in Disko Bay, on the west coast of Greenland, Le Vesconte surveyed ashore with his friend James Fitzjames, who recorded that Franklin was "much pleased with him".

After travelling down Peel Sound through the summer of 1846, Terror and Erebus became trapped in ice off King William Island in September 1846 and are thought never to have sailed again.

The will Henry Thomas Dundas Le Vesconte wrote is unusual in that it was actually written, witnessed, and signed aboard the Erebus on 15 May 1845 as he prepared to sail with Franklin just four days later.

[18] Le Vesconte appears as a character in the 2007 novel, The Terror by Dan Simmons, a fictionalized account of Franklin's lost expedition, as well as the 2018 television adaptation, where he is played by Declan Hannigan.

Le Vesconte's personal diary written in retrospect of his time on the China coast on board HMS Calliope, Cornwallis and Clio during the period January 1841 to October 1844 is in the collection of the National Maritime Museum.

When in March 1859 Francis McClintock and his expedition found a group of Inuit at Cape Victoria they retrieved a dessert spoon that similarly had been owned by Le Vesconte.

[24] A subsequent re-examination in 2009 of the "well-preserved and fairly complete skeleton of a young adult male of European ancestry"[26] included a facial reconstruction that showed "excellence of fit" with the face of Harry Goodsir, Erebus' assistant surgeon, as portrayed in his 1845 daguerreotype.

[27][28][29] Based on analysis of DNA with living descendants, the skeleton of an officer found on King William Island (sample NhLh-12:18) does not belong to Le Vesconte.