The class MJ1 was created in World War I when the SAR had found out that not only German manufacturers were now out of reach, but also British companies didn't have huge capacities. This became especially apparent with the 2-6-6-0 class MJ Mallet of which only two could be delivered by Maffei and another eight came from North British. So Montreal Locomotive Works designed the MJ1 based on the same specifications set up by D.A. Hendrie.

It was the last new class of Mallets introduced by the SAR. The MJ1 had a somewhat larger boiler compared to the MJ and had a sand dome mounted on the boiler in American fashion. The drivers of the eight locomotives were only 42 1/2 inches, but were also increased to 42 3/4 inches with new tires in 1922. In 1939, W.A.J. Day equipped this class and the MJ with a new common standard boiler. The last ones were retired in 1960.