The agency will replace McCann London, just five months after the Interpublic shop was appointed by HTV.
Mother is understood to have won a pitch against agencies including McCann to promote new HTC smartphones across Europe in the autumn.
McCann was appointed as a lead agency on the pan-European business in February this year. The agency won the business after a pitch that kicked off at the end of December 2010.
Mother joins Fearlessly Frank, Zenith Optimedia, Fever, Profero and Momentum on the roster. AMP London has lost its place on the HTC European roster.
Markus Breglec, HTC brand director EMEA said: "I can confirm that Mother is the latest addition to our integrated team of top class agencies."
The handset company is currently bolstering its European operations and has just hired its first UK and Ireland marketing chief. James Atkins, the former Freesat commercial marketing manager, took on the role last week.
HTC also poached Everything Everywhere's Phil Roberson earlier this month to become its executive director for UK & Ireland with responsibility for sales and marketing.
Ahead of McCanns’ appointment, HTC launched an international campaign in February through Amp to promote its two mass-market handsets, the "iPhone killer", Desire HD, and entry-level Wildfire.
The animated ad used the brand’s "quietly brilliant" tagline. The campaign ran in the UK, Europe, the Middle East and Asia, across TV, outdoor, press and digital media.