Sean Moran, executive vice president for linear and digital ad sales and integrated marketing for the music entertainment kids and family division, is replacing Lucas.
Moran will report to Tom Dooley, the chief operating officer, once Lucas leaves in the next couple of months, according to AdWeek.
Lucas will lead Snapchat entire sales operation, across offices in California, New York, London, Sydney and Toronto.
Lucas joined Viacom in 2006 as executive vice president for advertising sales and marketing for the MTV Networks Entertainment Group.
Moran has been with Viacom for 21 years having joined in 1995 as an account executive for MTV.
A Snapchat spokesman confirmed the move.