The digital distributor saw over five million copies downloaded in December 2011 alone, reports ICv2.
A "significant percentage" of the comics downloaded in December 2011 were free, but the specific proportion has not been revealed.
"This milestone shows the tremendous demand for digital comics from comiXology and the strength of our partnerships and reach, from device manufacturers to publishers to brick-and-mortar retailers," comiXology CEO David Steinberger said.
"And it shows the huge opportunities that exist to grow the comics market by bringing in new or lapsed readers that want a digital option for reading comics."
The comiXology app launched in July 2009 for iPhone and iPod Touch, expanding to the newly launched iPad in April 2010, the web in July 2010 and the Kindle Fire in December 2011.
> comiXology to support Amazon's Kindle Fire
> comiXology is attracting new readers, says CEO
Unlike Diamond Comic Distributors, digital distributors do not release a breakdown of their month-on-month sales.
It was estimated that the digital comics market was worth $25 million (£15.7 million) in 2011, tripling in value compared to 2010.