He was played by Bill Irwin.
He was first seen when he was shopping along with his younger daughter Cindy Lou and after they finish shopping, he was happy because he can't wait for Christmas and ask his daughter that nothing can beat Christmas which Cindy replied I guess.
After he ask Cindy why she say I guess, Cindy Lou told him that everyone is missing the point about Christmas by being more concerned about the gifts and festivities.
After Cindy's brothers are scared off by the Grinch when they trespass on his domain, Lou was having a conversation with the mayor and told everybody that his sons didn't see the Grinch.
Later, he took Cindy to work at the post office and they both talk about the grinch for a while. Then, he told Cindy to take the mail to the back room where she herself has a face-to-face encounter with the Grinch which he reluctantly saves her life. After the grinch left, Lou was looking for Cindy until he realize that she was wrapped up and thinking that she was making wrapping gifts.
Later, he and Cindy head home when Lou's wife Betty Lou Who was getting the lights all set up because of the light concert. So, he help her after Cindy head inside.
The next day on Christmas Eve, he and his family were heading to Whobilation where Cindy decides to make the Grinch the main participant of the Whobilation, much to the great displeasure of Mayor May Who, who reluctantly agrees after pressure from the townspeople, who have been warmed by Cindy Lou's generous spirit.
As the movie continues, the Grinch berates the Whos and criticizes Christmas, claiming that the holiday is only about gifts that they will just dispose of later, in the hopes of making them too ashamed to celebrate the holiday.
The Grinch then goes on to ruin the party by burning the Christmas tree with a makeshift flamethrower, but his actions prove to be fruitless as the Whos have a spare tree, which the Grinch sees them erect before he leaves. May Who then harshly scolds Cindy Lou for inviting the Grinch, making her feel ashamed.
The Grinch, knowing that his attack has failed to remove the Whos' Christmas spirit, instead concocts a plan to steal all of their presents while they are sleeping.
On Christmas morning, the Whos discover the Grinch's scheme, and May Who blames Cindy Lou for the whole disaster. However, Lou (who have enough with the mayor's attitude) finally stands up to him and reminds everyone that they still have Christmas spirit, and the true meaning of Christmas is to spend it with family and friends.
The people accept his fate and begin to sing. Hoping that the change of mood would inspire the Grinch, Cindy Lou goes to Mount Crumpit to find him.
After a long descent at Mount Crumpit, the Grinch returns to Whoville with Max, Cindy and the gifts after the Grinch redeemed himself. He confesses to the burglary, tearfully apologizes for his actions towards the Whos and surrenders himself to the police as they arrive, but the Whos reconcile with him, much to May Who's dismay.
Martha turns down May Who's proposal and decides that she would rather stay with the Grinch instead. The redeemed Grinch (along with Max) starts a new life with the Whos, commemorating the Christmas feast with them in his cave.