Longitudinal time-based emergency department (ED) performance measures were quantified 12 months before and 12 months after (March 2012–February 2014) implementation of a Meditech 6.0® electronic health record (EHR) at a single urban academic ED. Data assessed were length of stay from door to door, door to admission, door to bed, bed to provider, provider to disposition, and disposition to admission, as well as number of patients leaving against medical advice and number of patients leaving without being seen. Analysis of variance was used to compare levels before and after EHR implementation for each variable, with adjustments made for the number of admissions, transfers, and month. No difference was seen in monthly volume, admissions, or transfers. Implementation of an EHR resulted in a sustained increase in ED time metrics for mean length of stay and times from door to door, door to admission, door to bed, and provider to disposition. Decreased ED time metrics were seen in bed-to-provider and disposition-to-admit times. The number of patients who left against medical advice increased after implementation, but the number of patients who left without being seen was not significantly different. Thus, EHR implementation was associated with an increase in time with most performance metrics. Although general times trended back to near preimplementation baselines, most ED time metrics remained elevated beyond the study length of 12 months. Understanding the impact of EHR system implementation on the overall performance of an ED can help departments prepare for potential adverse effects of such systems on overall efficiency.