Mr. Walden is the father of Sally and Conrad and the husband of Joan Walden. He is seen in The Cat in the Hat Knows Alot About That Christmas Special helping Mr. Brown putting up a Christmas tree.
He is mentioned in The Cat in the Hat Comes Back by Conrad when The Cat in the Hat used his $10 shoes to wipe the cat ring spot from off the wall that was shook on there with Joan's dress, which he used to wipe it from the tub. He was mentioned again by Conrad as The Cat went into His and Joan's bedroom as The Cat shook the wrong of the rug spots on the bed where the little cats later get to work on cleaning the spot of the bed of his and his wife.
He does not appear in the 2003 movie adaptation of The Cat in the Hat, however when Larry Quinn points at the Joan raised Conrad and Sally on here own, this implies that Mr. Walden and Joan divorced or had passed away.