You can't really say they were used throughout the First World War, because only a handful were delivered before the end of 1917. By 1918 standards, it was a good tank, and produced in relatively large numbers (around 4,000, both French FT and US M1917 production). Consequently, in the post-WWI downsizing, they were sold to many different countries. Most of these were retired prior to 1939, but some remained in inventories in France (and in their colonies), Finland, Poland, and Yugoslavia.

By 1939, the FT was hopelessly obsolete. They were slow. The armor could be penetrated by .50 caliber and 20mm rounds. The gun lacked the penetration of the comparably-sized high-velocity guns, but also lacked the explosive power of the larger, low-velocity guns. They were used primarily for training purposes. The only reason it saw combat at all in WWII was out of desperation. In Poland, Yugoslavia and Vichy-controlled North Africa, for lack of better tanks, and in France, because their good tanks were mostly trapped north of the German breakthrough.