Ausf G entered production in December of 1942 and remained in production until March/April of 1945. It was the most numerous from all Sturmgeschutz III guns and some 7893 were produced by Alkett (Altmaerkische Kettenfabrik GmbH) and MIAG (Muehlenbau-und-Industrie AG). Production reached its peak when in 1944, 4013 Ausf Gs left factories. Alkett produced over 5000 Ausf, while MIAG began production in March of 1943 and produced some 3000 vehicles. Production numbers include 165 PzKpfw III Ausf M chassis used to produce Ausf Gs in 1943 and 173 PzKpfw III Ausf Ms converted to Ausf Gs by Alkett and MIAG in 1944. Ausf G was produced in four production series - chassis numbers 76101 to 77550, 91751 to 94250, 95001 to unknown and 105001 to unknown.

Ausf G used Ausf F/8's hull, suspension, engine and other components, while superstructure was modified. Superstructure was widened, its 30mm sides were sloped at 79 degrees, the roof was raised in the rear and its rear 30mm superstructure wall was mounted at 90 degrees. This provided more room for both the commander and loader.

The other changes included the layout of the roof, addition of commander's cupola with seven periscopes (each could be moved up or down) and sighting flap for binocular spotting telescope along with a 10mm machine gun shield in front of loader's hatch (from December of 1942 to April of 1944). Early vehicles had movable cupola (until October of 1943), mid vehicles had it mounted in a fixed position (until October of 1944) and late vehicles again had movable cupola with additional protective cover. Also, early vehicles had 50mm frontal hull armor increased to 80mm by addition of 30mm bolted armor plate, while vehicles produced since May of 1943 by Alkett and since October of 1943 by MIAG had 80mm frontal hull armor. Early models were also mounted with KFF2 (Kampfwagen Fahrer Fernrohr) driver's episcopes.

During the production, modifications were made to Ausf G. They included introduction of 80mm cast Saukopf (also known as Topfblende) mantlet in February of 1944; the coaxial MG in early 1944, installation of Nahverteidigungswaffe (90mm NbK 39 close-in defense weapon) and roof mounted remote controlled MG (Rundum Feuer) in late spring of 1944. In addition, vehicles produced since January of 1943 had the fighting compartment fan mounted on the rear superstructure wall instead of the roof. Two kinds of "Saukopf" mantlet were produced and mounted on Stug III Ausf G, one housing only the gun and other housing the gun and coaxial machine gun (from September of 1944). Original "boxy" gun mantlet was made of 50mm (front) and 30mm (sides) armor plates. Ausf Gs were also mounted with 5mm Schurzen since mid 1943.