Diane is shown to have been seen throughout all three films and the television series.
Diane was said to have been having problems with Wayne, so she spent the night at her mother's house. At the same time, Wayne was working on the experimental and somewhat non-operational shrinking machine. Diane sells a house before returning home and eventually discovers the kids are gone. Eventually, Wayne comes to her with the fact that the machine works, however unbeknownst to her at the time that the kids, as well as the Thompson ones, were shrunk.
Realizing that they are in the backyard shrunk, Diane decides to assist Wayne in repairing his machine. After finding poor Nick in Wayne's cereal, they discover the others and eventually get them up to the attic where they return them to normal size.
Two years after the incident with Nick and Amy, the Szalinskis move to Nevada and add have another child, Adam. Amy is going away to college and Diane accompanies her in order to get her settled in while leaving Wayne to take care of Nick and Adam while she's gone. She returns home early to find Adam's favorite toy, Big Bunny, overgrown in the house. Wayne tries avoiding a confrontation with her; however, when she sees Adam at an unnatural size, she faints. After coming to, she almost kills Wayne because of what happened and then assists both him and Nick into calming Adam down.
While Wayne is going through different solutions, he realizes the only way to reverse the process is his shrinking machine. Reluctantly leaving Adam in the capable hands of Nick, Diane accompanies Wayne to the Sterling Labs warehouse where they find the machine. She drives on the way home while Wayne works on rebuilding it on the road. Returning home, Diane finds that Nick and Adam, along with Adam's babysitter, Mandy Park, are gone. Meeting up with Wayne's boss, Clifford Sterling, and seeing Wayne's rival, Dr. Charles Hendrickson, being fired, they head toward Las Vegas where they try to stop Adam by using Big Bunny.
After Wayne uses the word "nap", Adam starts throwing a fit and ends up heading for Las Vegas. Diane drives most of the way, and assists in trying to stop him from causing any unwanted destruction in getting there. She and the others manage to stop an ice cream truck, which Marshal Preston Brooks uses to lure Adam out of Las Vegas, however, Adam manages to grab the giant ice cream bar off the top of the truck and bites a chunk out of it. Eventually, Diane comes up with the idea to enlarge herself and keep Adam still while Wayne shrinks them back to normal size. When noticing the helicopter carrying Dr. Hendrickson, she says to Wayne "Wayne, time to get large". Reluctantly, he uses his machine in order to enlarge Diane's size to an unnatural one.
While Dr. Hendrickson shoots his tranquilizer cannon on Adam twice, he makes him cry, and as a result, Diane grabs hold of the helicopter and says "Back off!". While putting it on the ground, Adam runs to her with open arms and she holds him while Wayne shrinks them back to normal size. Dr. Hendrickson arrives at the scene and congratulates Wayne for getting Adam back to normal size. Diane confronts Dr. Hendrickson about him shooting at Adam with "some sort of rifle," to which he says, "They were tranquilizer cartridges. They wouldn't have hurt him, I assure you." She pretends to buy it before turning back to Dr. Hendrickson and angrily punches him in the face, knocking him out cold.
After the incident with Adam, the Szalinskis have moved again and this time they live closer to her brother-in-law Gordon Szalinski, his wife Patti, and their two kids Jenny and Mitch. Before leaving on a two-day vacation with Patti, Diane leaves a list of instructions for Wayne and Gordon to follow. One such rule is for Wayne to get rid of his Tiki Man statue. Eventually, when they leave, Patti forgets to leave Mitch's medicine and as a result they have to go back.
After bringing Mitch's medicine to the house, Diane and Patti are about to leave when they hear the shrinking machine getting fired up in the attic. Upon entering, they find it on and no operators, however, that's until they go to leave and they're accidentally shrunk like their husbands. She gets angry due to the fact that she is smaller than a raisin and that she isn't even on vacation due to being shrunk. Eventually exiting the attic via a fishing reel, they end up in Adam's room where they discuss a plan to get their kids' attention via the stereo's speakers. Agreeing with it, they hitch a ride on Adam's Shark Cruiser Hot Wheels car which sends them falling down the laundry chute. They are then brought back upstairs right where they started and end up having to brave a cockroach in order to get to Jenny's friend Holly's bubble machine, which she brought for the party.
Diane and Patti end up in the kitchen where they encounter a Daddy Long Legs caught in cobwebs. While Patti cuts him free, Diane distracts him and they eventually hitch a ride on him as he crawls up on the counter. Eventually, Mitch becomes ill from potassium deficiency, and despite their attempts to get his medicine in his visual range, they decide to shout to get his attention. They succeed in doing so, but he faints and as a result, Jenny and Adam attempt to revive him. After they do so with bananas, he gets up, then Adam and Jenny go to investigate a disturbance in the living room. Eventually, Jenny and Mitch go to find Diane and Patti in order to bring them up to the attic where they're returned to normal size. After the events of them being shrunk, they return home after picking up Adam at baseball camp.
This movie establishes that Diane has died sometime after the events of Honey, We Shrunk Ourselves.