Performing time-critical procedures such as Cardiopulmonary Resuscitation (CPR) usually requires trained individuals on the scene. Even when step by step instructions are available, most bystanders do not attempt resuscitation due to panic or fear of failing, often at the cost of the victim’s life. We propose Mixed Reality (MR) as a compelling medium to support time-critical emergencies, and study its use in this context through an iterative user-centered design process. Our research outlines a number of key considerations for the design of time-critical emergency interfaces that led to the creation of HoloCPR, an MR application providing real-time instructions for resuscitation through a combination of visual and spatial cues. HoloCPR’s comparative evaluation during a realistic resuscitation scenario indicates how the use of MR can result in decreased reaction time and increased procedural accuracy. With this work, we hope to bootstrap a new wave of MR applications for time-critical emergencies that can be included in first aid kits in the future.