So, which engine was the first to use electronic controls? The Power Stroke, Duramax, or Cummins? Well, two of the engines, the Power Stroke and Duramax, were always electronically-controlled to begin with. Both of those engines replaced pre-existing indirect injection (IDI) engine lines.
The 7.3L Power Stroke, which kicked off the Power Stroke line in 1994, is a fully electronically-controlled turbo diesel engine that replaced the old all-mechanical 7.3L IDI (which used the Stanadyne DB-2 injection pump).
Also in 1994, although the Duramax wasn't yet introduced, GM's diesel option at that time, the 6.5L IDI, became electronically-controlled as its DB-2 pump was replaced with the Stanadyne DS-4 pump. The electronic trend continued in a big way with the Duramax from 2001 onward, which began using the CP3 common rail injection pump to kick off a major improvement in engine technology.
The Dodge/RAM Cummins line is the only engine line of the Big 3 to have used both mechanical and electronic controls. The 5.9L used the Bosch VE mechanical rotary fuel pump from its inception in 1989 up through the 1993 model year. From 1994 through early 1998, the engine used Bosch's P7100 inline injection pump, and then in mid-1998, the 12-valve engine became an electronic 24-valve engine with the implementation of the VP44 injection pump.
So, there you have your answers. Technically speaking, the Power Stroke was the first of the Big 3 to use electronic controls if you don't want to count the pre-Duramax GM diesels. From this information, you can see that the 1990s were a major decade for improvements in diesel technology.