Early in the novel, in part I, after Montag met Clarisse,he realizes that he and Mildred have grown apart, and he really isn't happy. Mildred had become a puppet for the state, and he had not bought into that idea yet. He realizes that,
"...she was so strange he couldn't believe he knew her at all" (pg 42)
He asks her when and where they met, and she responded that she didn't know. It has only been ten years, and yet she had no idea when or where they met. After he experiences the woman dying with her books, Montag tries to talk to Mildred about that experience, and he suddenly remembers how he felt when the medical technicians were bringing her back to life.
"...he rememembered thinking then that if she died, he was certain he wouldn't cry. For it would be the dying of an unknown, a street face, a newspaper image, and it was suddenly so very wrong that he had begun to cry, not at death but at the thought of not crying at death, a silly empty man near a silly empty woman, while the hungry snake made her still more empty." (pg 44)
He suddenly realizes that they have nothing in common. He tries to interest her in his books, but all she wants to do is hide and destroy them. She doesn't want to lose her house or her precious walls of talking people. When she finally realizes that Montag is not about to give up his idea of learning from books, she turns him in to the government, and as they start to burn down their house, she runs out and disappears. When the atom bomb drops at the end of the book, he thinks first of Faber, second of Clarisse, and third of Mildred.
"Montag, falling flat, going down, saw or felt, or imagined he saw or felt the walls go dark in Millie's face, heard her screaming, because in the millionth part of time left, she saw her own face reflected there, ....a wildly empty face, all by itself in the room, touching nothing, starved, and ating of itself...." (pg 159)
He did think of Millie when the disaster came around. He remembered where they met, Chicago. However, to have stayed with her would have shaken the foundation of what he believed. They had grown apart, and he wasn't going to miss her. He had moved on to a new experience in his life, an experience he embraced wholeheartedly.