Mother won the global account in March last year after a pitch against HTC's US incumbent, Deutsch LA, and its agency in Asia, Saatchi & Saatchi Singapore, after picking up the pan-European business in July 2011.
HTC's contract with Mother had come to an end and the company has now moved its global advertising business in-house to be run out of its headquarters in Taiwan.
HTC has been suffering financial woes lately, losing market share to rivals such as Samung.
Earlier this month, HTC announced that fourth-quarter operating income fell by 95% to just NT$0.6bn (£12m) on revenues, down by 41% to NT$60bn (£1.2bn).
Mother's last major global campaign for HTC featured TV and digital work showing the world's first free-fall fashion shoot filmed on a mobile phone.