[3] Halliday subsequently entered the University of Edinburgh and started training for the Presbyterian ministry, but switched to medicine, his preference.

[3] Whilst in the British Army, Halliday served in the Napoleonic Wars in Portugal, Spain, and the West Indies, at the Siege of Bergen op Zoom (1814) and the Battle of Waterloo.

[8] Halliday was the first physician to the Seamen's Hospital Society,[10] which was established in 1821 with the purpose of helping people currently or previously employed in the Merchant Navy or fishing fleets.

[4] He wrote Annals of the house of Hanover and The West Indies: the Nature and Physical History of the Windward and Leeward Colonies, published in 1826 and 1837, respectively.

[4][5] His nephew, Andrew Smith Hallidie, promoted the Clay Street Hill Railroad in San Francisco, the world's first practical cable car system.