Objective: To describe the numbers and length of stay (LOS) of patients with mental health (MH) problems at a Dutch emergency department (ED) and the effect of a psychiatric intervention team (PIT) on patient flow. Methods: A longitudinal design was used to assess number of MH presentations and LOS during a 3-year period (2014–2016). In 2017, we introduced a PIT during ED peak hours, to reduce LOS for patients with MH problems. We evaluate the effects of the PIT on patients’ LOS with an 18-month before and after intervention study (2017–2018). Results: Total number of ED presentations increased with 4%. Total number of MH presentations increased with 23% from 2014 to 2016. LOS increased by 28 min (95 min vs. 123 min) for all presentations, while not changing for MH presentations (2014: 195 min, interquartile range (IQR) 120–293 and 2016: 190 min, IQR 116–296). In the before and after intervention study, number of MH presentations increased with 36% while LOS decreased with 46 min (p < 0.001). Conclusions: The number of MH presentations increased over the three years while LOS remained similar. In the before and after intervention study, number of presentations increased even more while LOS decreased significantly. Specialist psychiatric input reduces ED LOS.