The American actor was found dead in his LA home on Tuesday morning after reportedly suffering a relapse in his battle against drugs.
McKellen starred alongside Renfro, then 15, in the 1998 adaptation of Stephen King's novella Apt Pupil, about a teenager who discovers a Nazi war criminal living nearby.
He wrote in his blog: "I first caught sight of Brad Renfro when he was kicking a football around with Bryan Singer on the half-built set of Apt Pupil in Hollywood. He was a kid having fun and that's how I shall always remember him.
"But he was more than that. He was a proper actor and when we worked together he was determined to be accepted as such. On set, he was blusteringly confident, although it was obvious he would have benefited from training as an actor. Yet, as Todd, the disturbed teenager in Apt Pupil, he tapped into an inner demonic world and carried the film on his young shoulders."
McKellen added: "He longed to belong in the alien world which perhaps in the end overwhelmed him. He was only 25 and it is dreadful we shan't see all that he might have achieved."