1945: "Correspondence Course", a short story by Raymond F. Jones, first published in Astounding Science Fiction, April 1945, available at the Internet Archive.
We don't get to see the symbiosis in action, as the story ends right after the human protagonist accepts the alien symbiote, a shipwrecked space traveler whose symbiotic partner has died.
"I'll bargain with you," said Quilcon at last. "Let me be the other of you, and I'll give you what you want."
"The other of me? What are you talking about?"
"It is hard for you to understand. It is union—such as we make upon our world. When two or more of us want to be together we go together in the same brain, the same body. I am alone now, and it is an unendurable existence because I have known what it is to have another of me.
"Let me come into your brain, into your mind and live there with you. We will teach your people and mine. We will take this ship to all the universes of which living creatures can dream. It is either this or we both die together, for too much time has gone for me to return. This body dies."
Stunned by Quilcon's ultimatum, Jim Ward stared at the ugly slug on the wall. Its brown body was heaving with violent pulsations of pain and a sense of delirium and terror came from it to Jim.
"Hurry! Let me come!" it pleaded.
He could feel sensations as if fingers were probing his cranium looking, pleading for entrance. It turned him cold.
He looked into the years and thought of an existence with this alien mind in his. Would they battle for eventual possession of his body and he perhaps be subjected to slavery in his own living corpse?
He tried to probe Quilcon's thoughts, but he could find no sense or intent of conquest. There were almost human amenities intermingled with a world of new science and thought.
He knew Quilcon would keep his promise to give the secrets of the ship to the men of Earth. That alone would be worth the price of his sacrifice—if it should be sacrifice.
"Come!" he said quietly.
It was as if a torrent of liquid light were flowing into his brain. It was blinding and excruciating in its flaming intensity. He thought he sensed rather than saw the brown husk of Quilcon quiver in the hemisphere and shrivel like a brown nut.
[. . . .]
The solitary figure of Jim Ward moved toward the ramp and disappeared into the depths of the ship.
1935: "Parasite", a novella by Harl Vincent, published in Amazing Stories, July 1935, available at the Internet Archive. Actully, I don't think this story meets your requirements, but I leave it up to you. It depends on what you mean by "mutually beneficial". The invading "puppet masters" do impart "enormous strength and vitality" to their human carriers, so there's that. 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 converted a small group of their leaders into conscious beings formed of electricity. They then altered the remainder of the population into small seed-like germs that could be reanimated at the proper occasion. The aliens then inserted electric beings and germs into a small metal sphere and sent it across space to Earth, which they knew from their advanced science would be suitable for them.
When the space vessel crashes as a meteorite in northern New Jersey, it releases the thirty-four electrical beings, who seize upon suitable humans as carriers. Invisible, since they are pure force, the aliens fasten upon the back of a human's neck and are absorbed into the human nervous system, imparting enormous strength and vitality. The human carriers are conscious, do not suffer any pain, but are no longer in control. The leaders of the migration, suitably housed, now are ready to awaken their fellows and let them loose upon humanity.
