The agency won the business after a pitch and will now create a TV and digital campaign for Admiral that will break later this summer.
Admiral called the review in December last year and appointed the AAR to oversee the process.
CST's appointment marks the first time that the Admiral Group-owned brand has held a relationship with an external creative agency. It has previously used an in-house marketing team to create its ad campaigns.
MPG handles the brand's media account.
Recent creative work has featured the brand's spokesman "the Admiral", who advises consumers on how they can find Admiral's best deals.
Other campaigns have featured Sir Steve Redgrave.