Mobile authentication methods protect smartphones from unauthorized access, but also require users to remember and frequently enter PINs, passwords, or graphical patterns. We propose the EmojiAuth scheme with which we study the effects of Emoji use on the usability and user experience of mobile authentication. We conducted two between-subjects studies (lab study: n = 53; field study: n = 41) comparing EmojiAuth to standard PIN entry. We find that EmojiAuth provides good memorability for short passwords and reasonable memorability for longer passwords. Moreover, we identify diverse Emoji-password selection strategies and provide insights on the practical security of Emoji-based mobile authentication. Our results suggest that Emoji-based authentication constitutes a practical alternative to traditional PIN authentication.