What if Spider Man kept the symbiote?

What is the first example of an intelligent symbiotic organism before Marvel Comics?

1945:"Correspondence Course", a short story by Raymond F. Jones, first published in Astounding science fiction , April 1945, available from the internet archive.

We can't see the symbiosis in action as the story ends right after the human protagonist accepts the alien symbiote, a shipwrecked spaceman whose symbiotic partner has died.

"I'll negotiate with you," said Quilcon at last. "Let me be the other of yours and I'll give you what you want."

"The other of me? What are you talking about?"

"It's hard for you to understand. It's a union - like we do on our world. If two or more of us want to be together, we go together in the same brain, in the same body. I am alone now and it is an unbearable existence because I knew what it was like to have someone else from me.

"Let me get into your brain, into your mind, and live with you there. We will teach your and my people. We will take this ship to all universes that living things can dream of. Either this or we both die together. I have to took a long time to return. This body is dying. "

Amazed by Quilcon's ultimatum, Jim Ward stared at the ugly snail on the wall. His brown body was full of violent pain pulsations and a feeling of delirium and terror came from Jim.

"Hurry up! Let me come!" it pleaded.

He could feel sensations as if fingers were examining his skull and asking for entry. It made him cold.

He looked back over the years and thought of an existence with this alien spirit in his own. Would they fight for possible possession of his body and he could be subjected to slavery in his own living corpse?

He tried to examine Quilcon's mind, but could find no sense or purpose of the conquest. There was almost human comfort mixed with a world of new science and thought.

He knew Quilcon would keep his promise to tell the men of the earth the secrets of the ship. That alone would be worth the price of its sacrifice - if it were to be sacrifice.

"Come!" he said softly.

It was like a stream of liquid light pouring into his brain. It was dazzling and excruciating in its flame intensity. He thought he could feel rather than see the brown shell of the quilcon quiver in the hemisphere and shrink like a brown nut.

[. . . .]

The lone figure of Jim Ward walked towards the ramp and disappeared into the depths of the ship.

1935:"Parasite", a novella by Harl Vincent, published in Amazing stories , July 1935, available from the Internet archive. Actually, I don't think this story fits your needs, but I'll leave it up to you. It depends on what you mean by "mutually beneficial". The invading "puppet masters" impart "tremendous strength and vitality" to their human wearers, so there it is. Here is an excerpt from Everett F. Bleiler's review in Science fiction: The Gernsback Years :

When the humanoid beings in a system about twelve light years away learned that life would no longer be possible on their planet, they transformed a small group of their leaders into conscious beings made of electricity. They then turned the rest of the population into tiny seed-like germs that could be revived at the right opportunity. The aliens then inserted electrical beings and germs into a small metal ball and sent them across space to Earth, which they knew from their advanced science would be suitable for them.

When the spaceship crashes as a meteorite in northern New Jersey, it releases the thirty-four electrical beings that suitable humans seize as carriers. Invisible as it is pure power, the aliens attach themselves to the neck of a person and are absorbed by the human nervous system, which gives enormous strength and vitality. The human carriers are conscious, not in pain, but no longer in control. The leaders of the migration, properly housed, are now ready to awaken their companions and let them loose on humanity.