Missing transverse momentum ($E_{\rm T}^{\rm miss}$) is a useful way to signal particles not interacting with the detector such as those predicted by Beyond Standard Model theories. The ATLAS experiment at the LHC employs a missing transverse momentum trigger that uses calorimeter-based global energy sums, together with specifically developed pile-up mitigation techniques, to keep the selection efficiency high for events with non-interacting particles while minimizing the overall trigger rate. The high number of pile-up interactions was one of the major challenges faced during the Run 2 of the LHC and a continuous effort was needed to improve the pile-up rejection and to keep the trigger rate reasonable. This contribution presents the techniques used to improve the Run 2 missing transverse momentum trigger performance, describes the Run 2 performance and discusses an outlook for further improvements for Run 3 of the LHC.