Preheat the oven to 300 degrees Fahrenheit. Add a silicone mat or sheet of aluminum foil to a rimmed baking sheet, then spread the rice cereal out over the baking sheet. Set aside.
Add the brown sugar, corn syrup, and butter to a small saucepan over medium heat. Stir continuously until the mixture is melted and smooth, then remove from heat.
Pour the mixture over the rice cereal and gently stir until each piece is coated. Spread the rice cereal out into an even layer. Bake in the preheated oven for 30 minutes, stirring every 10 minutes.
Place a second silicone mat or sheet of aluminum foil lightly sprayed with cooking spray onto a cooling rack. Pour the rice cereal out over the mat and allow to cool fully.
Once the caramel cereal is fully cooled, combine the cereal, pistachios, cashews, raisins, and M&M;'s in a large bowl. Gently stir to combine.
Store in an air-tight container for up to 3 days or in the freezer for up to a month.