Back in December, we reported that Google was adding a mobile usability feature to Webmaster Tools. The feature is designed to help webmasters determine which pages on their site are not mobile-friendly, and it provides suggestions for fixing usability issues.
At the time, it was questionable whether or not Google considered mobile usability to be a ranking factor. Now, however, the search giant has sent emails to webmasters directly indicating that pages not considered mobile-friendly will be "displayed and ranked appropriately." This makes it clear that mobile usability will affect rank.