The first completely 64-bit compatible version of Android was Android 5.0 Lollipop.
Android 5.0 Lollipop
Android Lollipop is the fifth major version of the Android mobile operating system. It was initially released on June 25, 2014. Android 5.0 Lollipop is developed by Google and the 12th version of Android, spanning versions between 5.0 and 5.1.1.