The Cornwall-Massena earthquake (MW 5.8) of 5 September 1944 was one of the most damaging earthquakes in Canadian history. Its epicenter, near the Canada-United States border and the southern end of the Western Quebec seismic zone, lies within a heavily populated region, rendering this earthquake significant for regional seismic hazard analysis. The reappearance of a seismogram collection from the 1940s previously unavailable to (recent) researchers helps provide new insight into this earthquake. Waveforms recorded at regional distances indicate that this was a mid-crustal (20-km deep) oblique thrust event (strike 313°, dip 70°, rake 52°). Either nodal plane is a plausible fault plane in terms of the local and regional seismotectonics—one is more consistent with the seismicity pattern and the other with the mapped faults. The mechanism is similar to those determined for many smaller and more recent events in the Western Quebec seismic zone and is consistent with the regional stress field. The low value of a previously determined mb (4.6) relative to the moment magnitude is at least partially a result of the P-wave radiation pattern and the distribution of stations used to calculate mb. Previously calculated values for MS range from 5.1 to 5.6. While the larger number is more consistent with the MW determined in this study, the reason for the large variation in MS values has not yet been fully explained.