Pairing grilled cheese with tomato soup first emerged in the early 20th century, between the Great Depression and post-World War II. It was created by way of its pragmatic nature.
Toasted cheese sandwiches were popular in the early 20th century as an economic recession meal and popular military ration. Tomato soup followed, and the combination caught on.
The soup nicely complemented the sandwich and added some vitamin C. The pair was a hit with bureaucratic institutions, cafeterias in schools, and governmental organizations.