The agency won the business after a pitch involving EHS Brann, Rapier, Tequila\ and Kitcatt Nohr Alexander Shaw.
The pitch, which was first called in May, was led by COI.
TMW will now create DM campaigns to raise awareness of the importance of giving blood, and attract prospective new donors.
The agency will work closely with Delaney Lund Knox Warren, which the National Blood Service appointed to handle its above-the-line account last year.
In the past, Kitcatt Nohr Alexander Shaw has handled the majority of the National Blood Service's campaigns. The agency was first appointed to the business in 2002, however the account has laid dormant for the past two years.