A mixer is disclosed for use in a fluid injection system. The mixer may have an impingement floor oriented generally perpendicular to an intended fluid injection direction and generally parallel with a flow direction. The mixer may also have a first side wall connected along a lengthwise edge of the impingement floor and generally parallel with the flow direction, and a second side wall connected along an opposing lengthwise edge of the impingement floor and generally parallel with the flow direction. The mixer may further have at least one shelf extending between the first and second side walls and generally parallel with the flow direction. The at least one shelf may have a plurality of vanes protruding toward and away from the impingement floor that promotes mixing of an injected fluid.