High-angular-resolution cosmic microwave background experiments provide a unique opportunity to conduct a survey of time-variable sources at millimeter wavelengths, a population which has primarily been understood through follow-up measurements of detections in other bands. Here we report the first results of an astronomical transient survey with the South Pole Telescope (SPT) using the SPT-3G camera to observe 1500 square degrees of the southern sky. The observations took place from March to November 2020 in three bands centered at 95 GHz, 150 GHz, and 220 GHz. This survey yielded the detection of two long-duration transient events from sources not previously detected by the SPT. Each event was measured to have a triangular lightcurve lasting 2-3 weeks and appear to be extragalactic in origin. Future data from SPT-3G and forthcoming instruments will provide real-time detection of millimeter-wave transients on timescales of minutes to months.