fremantle was born into a distinguished military family; his father, lieutenant-general john fremantle, had commanded a battalion of the coldstream guards, and had served during the peninsular war and waterloo campaign, as well as acting as aide-de-camp to lieutenant-general john whitelocke during the abortive british invasion of buenos aires in 1807. arthur's middle name, lyon, came from his mother, agnes lyon. he was called "arthur" after the duke of wellington, who had been the first witness at his parents' wedding in 1829.