The game has earned TellTale an estimated $40 million, with the average revenue per user valued at around $16.
"To date, it's sold 8.5 million episodes and it's been in the marketplace for seven months," Telltale CEO Dan Connors told The Wall Street Journal.
"November and December were our biggest months. It was the culmination of the product, the game of the year awards, the retail push, and it still has a ton of head room in."
The episodic adventure game has been particularly successful on mobile platforms, achieving 25% of total revenue on iOS devices.
"The episodic model has allowed us to create an economy that works on mobile," Connors added.
"As far as size, at this point it's probably about 25% of the revenue of the product and it is on the largest upswing of all platforms as well.
"I think we see with iOS products, we see more impact from when the show is on, when there's stress about the show."
Connors went on to say that the game made use of TellTale's work on other franchises, combining the dramatic storytelling found in Jurassic Park with Monkey Island's cliffhangers.
"The Walking Dead benefits from the work done on all the previous products. Everything has been an investment in creating great storytelling and great dramatic experiences that work across multiple platforms," he explained.
"Cost-wise, it's 40% more expensive than other games, but it's not huge."
Episode 1: A New Day introduces main character Lee Everett, who encounters the zombie outbreak while on his way to prison on murder charges. Soon he finds himself protecting the 8-year-old girl Clementine, both from zombies and from other survivors.
The Walking Dead was recently voted Digital Spy's 2012 game of the year, beating the likes of Far Cry 3, Black Ops 2 and XCOM: Enemy Unknown.
> Why 'The Walking Dead' is Digital Spy's 2012 Game of the Year
> Game of the Year 2012: Digital Spy picks the most memorable moments
Photo gallery - Digital Spy's 2012 Game of the Year: