Hanuman in Mahabharat

 Hanuman is the one among 7 immortals who was greatest devotee of Ram. He was incarnated by lord Shiva in Treta Yug. God Ram had finished  main ambition of his birth and left the earth. But Hanuman couldn't go with his idol because he is immortal (he can't die).

In Dwapara yug, Hanuman ji somehow knew that Shri krishna is reincarnation of ram and Balram is of Laxman. Hanuman ji said to Shree krishna " Prabhu, you gave me lots knowledges in your Ram form and I need more of that in your Krishna form also". God Krishna smiled and replied to him that " Hanuman you are my biggest devotee and I can't avoid you my child ". 

In the battle of Mahabharata, Hanuman ji sat on the flag of chariot of Arjuna and God Krishna by minimizing his body( extremely small). During the war, Krishna was giving Geeta gyan to arjuna but his main target was Hanuman. The knowledges of Geeta was took by arjun along with Hanuman.

In fight between Karna and Arjuna, the chariot of Karna moved 4 step backward after Arjuna hit his arrow but only 1 step backward of Arjuna after Karna hits. Shri Krishna was satisfied with Karna target only and eventually Arjun asked to lord Krishna "Hey Keshav, why you are frequently pleased to Karna only even though I moved his chariot 4 step backward and he did only 1 step". Shri Krishna said " there is answer in your question. He is moving the chariot in which Hanuman is sitting. Now imagine how much he is strong and his power."

Before the war, Bheem also met with Hanuman who was sleeping  and blocking the road with his tail while bhim was walking somewhere. Bheem tried to carry Hanuman's tail but he couldn't do it. 

In this way, Hanuman ji is maintioned in both Ramayana and Mahabharata. He is one of the 7 immortals (7 Chiranjivis).