It's hard to confirm the stories but there are several stories where Mithridatism actually saved an individual.
One of the famous stories of them is about the emperor Chandragupta Maurya. According to the Jain work Rajavalikatha,
Chandragupta's Guru and advisor Chanakya used to feed the emperor with
small doses of poison to build his immunity against possible poisoning
attempts by the enemies. One day, Chandragupta, not knowing about the
poison, shared his food with his pregnant wife, Queen Durdhara, who
was seven days away from delivery. The queen, not immune to the
poison, collapsed and died within a few minutes. Chanakya entered the
room the very time she collapsed, and in order to save the child in
the womb, he immediately cut open the dead queen's belly and took the
baby out, by that time a drop of poison had already reached the baby
and touched its head due to which the child got a permanent blueish
spot (a "Bindu") on his forehead. Thus, the newborn was named
In many Indian epics and historical books, a group of maidens called vishakanyas (visha = poison, kanya = maiden) is mentioned. Many kings was said to create a squad of vishkanyas to kills rival kings and high ranking official. The squad is made by selecting beautiful girls in their childhood and slowly poisoned them to create immunity to poisoning inside them. It was said that only 3 out of 10 girls is survived to their adulthood. Then they were sent to rival kingdoms. Vishkanyas first tried to seduce the king. When the king calls them to make love with him, they go and mixed the poison with the drink or food presented there. Then they eat the poisonous food to give the trust to the king that the food is not poisoned and give it to the king. This was the most commonly used tactic of the vishkanyas.
There are stories about Russian mystic Rasputin's survival of a poisoning attempt. Some suggested he survived poisoning due to the practice of mithridatism.