Electromagnetic ion cyclotron (EMIC) waves can act as a loss process for both ring current ions and radiation belt electrons, and the spatial and temporal characteristics of these waves are important for quantifying their effects on energetic particles. Here we utilize observations from multiple spacecraft to constrain the azimuthal and radial dimensions as well as the duration of an EMIC wave event occurring on the nightside of the inner magnetosphere on 7 July 2013. These combined observations reveal waves limited to a narrow radial extent but persisting ~10+ hr and spanning ~12 hr in local time. The solar wind conditions, geomagnetic activity, and plasma environment are also examined to better understand the conditions under which persistent nightside EMIC waves can occur. Relativistic electron phase space density profiles during this event reveal local minima concurrent with the wave activity, consistent with EMIC‐driven scattering and loss of radiation belt electrons. Plain Language Summary Various oscillating electric and magnetic fields, or waves, can interact with high‐energy particles in near‐Earth space and cause a change in the particles' energy and/or direction of motion. Where and when these waves occur can have a significant impact on how they interact with particles. Here we combine measurements from multiple spacecraft around the Earth to study one specific wave mode. While these waves are often thought to be localized and of short durations, we observe an event on 7 July 2013 where waves persist for over 10 hr and span the entire nightside of the Earth. We explore the dynamics of the Earth's magnetic field, in response to activity on the Sun, to better understand what causes these widespread, long‐lasting waves. Changes in the energetic particle environment around the Earth are also presented to examine the effects of these waves. Events like these have the potential to cause significant effects in the particle populations around the Earth. Key Points Sustained, azimuthally extended EMIC waves are observed spanning the nightside of the inner magnetosphere on 7 July 2013 Wave structure lies at the inner edge of the plasma sheet overlaping the outer plasmasphere and plumes following enhanced Kp activity Local minima in phase space density are observed concurrent with wave activity, consistent with EMIC‐driven MeV electron scattering