Any antagonist in Jean George's My Side of the Mountain would be considered any character who tries to thwart Sam's plans to live successfully on his own in the wilderness.
One of the characters who tries to thwart him is the fire warden. One day, Sam awakens to see from his tree a man pacing outside. As the morning sun grows brighter, Sam is able to identify the man as a fire warden by his uniform and badge. Sam then realizes that it has not rained in three weeks, and the fire planes "circling the mountains and valleys" in search of forest fire threats must have seen the smoke from Sam's fires and sent a fire warden to look at the site (p. 48). Sam regrets the fact that he had left so much evidence of his living their in front of his tree; however, he manages to evade the fire warden that day, and by the time he returns to his home in the tree, he sees that the fire warden has left. After that moment, he is careful to make fires using dry wood so as not to create a lot of smoke.
A second antagonist is a conservation officer named Leon Longbridge who appears later on in the book. The officer announces that he is there because he has heard that Sam is "harboring an endangered species--a peregrine falcon," which "caries a fine and a year's imprisonment" (p. 190). The man confiscates Frightful, saying she must be bred in captivity to propagate the species. Sam is devastated since he relies on Frightful for help with hunting and worries about how he will provide for his sister during the winter now that she has come to live with him for the year in her own tree. However, Sam eventually uncovers a scheme to sell falcons for profit and gets Frightful back.