Dorey joined Tremor at the beginning of the year as publisher development director, but left the company last week to join French start-up Criteo.
Clare joins from 3VC Media, the video ad specialist he co-founded. Prior to this he was at 24/7 Real Media, where he was senior sales manager for technology.
His job at Tremor is to expand the company's reach among UK publishers.
Adrian Lacey, managing director of Tremor, said: "Michael's recent experience running his own company in the video sector makes him a great hire as we continue to establish Tremor Media's UK presence in the video advertising sector."
Tremor Media makes it possible to include video in banner and other online advertising formats.