Mix all ingredients in a bowl, then use immediately. Or store the fajita seasoning in an airtight container for up to 3 months.
Tomato powder is optional, but provides a slight tang that is reminiscent of a fajita marinade. Fajita marinades often use lime to soften the skirt steak used for fajitas. If you don't prefer the sourness/tang of tomato, omit if from the recipe.