The agency will support MOO with thought leadership and live event hosting as it expands its offering from business cards to branded business stationery in a bid for a greater share of the $640bn printing industry.
The account will be led by Third City senior consultant Henry Warrington, who will report to MOO UK and Europe PR manager Celia Venables.
Venables said: "We love to work alongside businesses of all sizes to help them craft beautiful products that bring their individual brand stories to life.
"Third City really tapped into that in its pitch. It is a highly creative agency, with a strong strategic backbone to its ideas, and we look forward to the partnership we have ahead of us."
MOO was founded in Shoreditch in 2006 and today has a global workforce of more than 270 staff.