The couple have already sent out invitations to their upcoming nuptials on August 11, reports Celebuzz.
A source said: "The wedding invite is very Zach. While it has the formal and standard details of who, what, when, where - including Zach's full name, Zachary Knight Galifianakis, and his fiancée's, Mary Quinn Lundberg - it is also very funny.
"Zach included a drawing of himself and his bride-to-be. He made sure he looked like a little troll, while Quinn is a stunning tall blonde princess."
Lundberg is the co-founder of Growing Voices, a non-profit organisation that connects international donors to community-based projects.
Galifianakis appeared in the last two Hangover movies and will reprise his role as Alan in the third instalment of the franchise.
Next up for the 42-year-old actor is the political comedy The Campaign opposite Will Ferrell, scheduled for an August 13 theatrical release in the US and a September 28 release in the UK.
Watch a promotional video for The Campaign below: