In the beginning, Brutus was a street rat, no different from other rats. He, along with several other rats, was captured on behalf of the scientific organization NIMH, and given injections which both prolonged his life and improved his mental capacities.
Like the other NIMH Escapees, Brutus was freed fro his cage by fellow Uplifted Rat Nicodemus, who had first managed to decipher the instructions to open the cage. Like the other Uplifted Rats, Brutus settled down in an underground lair on the Fiztgibbon Farm.
Thanks to his great physical strength and his proficiency at using the Electric Spear, Brutus was chosen by his fellow Rats as the guard of the gate of the Rosebush Colony, keeping non-Uplifted rodents and other animals away from the Rats of NIMH's home. Presumably following Jenner and Sullivan's line of thought that other animals, even those who had been partially uplifted thanks to the Power emanating from the rats, were just animals, Brutus was harsh and even downright cruel in his defense of the gates, and nearly killed Mrs Brisby when she visited the Rosebush Colony in 1982.