The decision follows a review in which several agencies, including incumbent Havas' Arnold Worldwide, competed for the business. A Hartford spokesman declined to name the other agencies involved.
Havas sibling MPG and Arnold will continue to buy media for the marketer.
Campbell Mithun will break its first work for the marketer, a campaign for Hartford Mutual Fund during the NCAA basketball tournament in March.
The Hartford spokesman said the company will retain its 194-year-old stag logo, and that he does not expect spending on measured media to change following the appointment.
The Hartford's recent advertising has focused heavily on mutual funds, though the company, which had 2003 revenues of $18.7 billion, offers a wide range of investment and insurance products.
The company spent $10.8 million in measured media for the first seven months of 2004, according to TNS Media Intelligence/CMR.