The firm's work with Mendes was confirmed this week. The Canadian artist is just 18, but has already had two Top 10 UK chart hits.
The agency will work to promote Mendes' next single, his O2 Arena show in London next year, and performance at the Capital FM Jingle Bell Ball next month.
DawBell's work across print, digital and broadcast media with British magician Dynamo - real name Steven Frayne - began at the end of October.
Founded in 2009 by Polydor Records' Richard Dawes and The Outside Organisation's Stuart Bell, the agency's clients also include James Corden and 2016 signings Harry Styles and Rihanna.
The firm has also recently been re-signed by both The Brit Awards and the Isle of Wight Festival to provide PR for the two events in 2017.