Mike Delfino is a man who posed as the friendly neighborhood plumber, but who was actually working to find his former girlfriend (Deirdre Taylor) who had been missing for years and whose father, Noah, was dying. It was discovered that the toy chest that was dug up from the Young family pool contained the remains of Deirdre. Delfino was at one point convicted of dealing drugs and committing manslaughter when he accidentally killed a corrupt cop who was forcing Deirdre to have sex with him to keep quiet about her drug addiction. Deirdre, not Mike, was the drug dealer.
Mike is Zach's biological father. In the process of his investigation, Mike struck up a romance with Susan Mayer. The relationship hit shaky ground upon the discovery that Mike was Zach's father. Susan was understandably afraid of Zach after having been held up by him, and she also felt the need to protect her daughter from Zach. For these reasons, Susan not only encouraged Zach to head to Utah, but gave him the money to get there as well. When Mike discovered that Susan sent Zach to Utah, he ended their relationship.
After Edie Britt sets Susan Mayer's home on fire, Mike helped Susan obtain a wire to convince Edie to confess. Mike then asks Susan if she would consider moving in. Susan gives the thought consideration and leaves Mike's house. Later they make up and he wants to propose to her in the second season's finale, but on his way he is hit on purpose by a car driven by the dentist Orson Hodge.
Mike survived, but was in a coma from which he woke up 6 months later but suffered retrograde amnesia, losing the last 2 years of his past. Having forgotten his relationship with Susan Mayer, Mike had new, distorted ones suggested to him by Edie Britt.
He was soon arrested as Monique Polier's (whom he somewhat remembers) blood was found on his wrench and subsequently dumped by Edie. Since then Susan and Ian are doing the best they can to get him out of jail. In prison he is found by Paul Young who attempts to befriend him in order for Zach to contact him and try to find Felicia Tilman, of whom he was accused of murdering at the end of Season Two.
Later he visited a hypnotherapist to try to remember who murdered Monique Polier. Flashbacks helped him regain all of his memory, and Mike confronted Orson Hodge, and fought with him. During this fight, Orson attempted to evade Mike and fell off the hospital roof. The police have to release Mike when they find Alma and Monique's teeth and suicide note. Mike witnesses Ian propose to Susan and is saddened when she accepts.
Later Mike learns that Ian had seen the engagement ring, with which Mike was going to propose to Susan, the night he got hit by a car and says to Ian he knows Ian only proposed to Susan so early because Susan may learn the truth. They make a deal where if Mike wins the poker game they are playing Ian will allow Mike to tell Susan anything but if Ian wins Mike cannot say anything to Susan and he is to leave them alone. Ian wins and Mike cannot tell Susan anything.
Nevertheless, despite his loss, Mike's feelings for Susan continue to grow. He is constantly seen as a threat by Ian, especially after Mike saves him from their car, which was overturned in a lake. After a heated argument between Ian and Susan, she walks over to Mike's for consolation, and they share an intimate kiss. This renews Mike's belief in the possibility of winning her back, and so he continually forces his way into her and Ian's lives, even confessing to what occurred at the poker game. After learning about this, Susan sends both of them away and cancels her wedding with Ian.
Ian and Mike continued to pursue Susan, but were shot down with each attempt. Eventually, Susan announced that she would go through with her marriage to Ian and she explained to Mike her reasons for this. Upset by this, Mike left a Dear John Letter on her voicemail saying goodbye. Under the cover of the night he left Wisteria Lane and Susan (who had been dumped by Ian because of her continued love of Mike) behind.
Mike is on a camping trip, so Susan goes to the woods to find him. She and her wilderness guide, Toni, get lost in the woods. Susan complains to Toni about her complicated love life, and Toni yells at Susan and tells her that she doesn't think Susan can handle a permanent relationship because she invites in too much drama. Susan decides to sneak off while Toni is sleeping and try to find Mike on her own. Back at the campground, the one hiker is reported lost, and Mike recognizes Susan from her description. Susan leaves a message on Mike's cell phone promising him that if he takes her back that she will not make their relationship dramatic and they will be a normal couple. In the morning, Mike finds Susan and picks her up and they kiss.
Mike and Susan go out to dinner, the night exactly a year after he and Susan were meant to become engaged and Mike was run over. Susan waits for Mike to pop the question all night, but Susan’s disappointed when he doesn't. On the way home, Susan tells Mike what she expected and when they pull up into Wisteria Lane the trailer Susan decorated on the night they intended to get engaged was waiting on her driveway. They live the experience they were meant to have that night and they get engaged.
Mike later marries Susan in the woods in the middle of the night. Susan surprises him with a small wedding because she doesn't want to put pressure on Mike to give her an expensive wedding.
In the first episode of Season Four, Mike and Susan find out from the new neighbour and doctor Adam Mayfair that Susan is pregnant and Mike tells Susan he has never been happier. In the second episode of Season 4 "Smiles of a Summer Night" Julie asks Susan if she can go to a party, however Mike tells her he'd rather her not go due to the fact he's seen some crazy parties in the house in question. Even though Susan had given Julie permission to go to the party she sided with Mike and Julie was banned. However Susan decided to go behind Mike's back and tell Julie she just wanted Mike to feel part of the decision making process and she allowed Julie to attend the party. Later that night Mike tells Susan he drove past the house earlier on and saw things like girls running around topless, people staggering around drunk and music blaring. Susan said she needed to run to the store and she went to the party to pick up Julie. However some boys at the party mistaken her for a stripper but she told them she was pregnant and she took Julie and Dylan who was with her home. Mike later found out and he told Susan that he respects her decisions on how to raise Julie how she wants as it's her child.
Susan soon accidentally learned that Mike's father was alive, having previously been told by Mike that he was dead. Mike told Susan that his father was in prison for first degree murder. Mike took Susan to see his father in prison where she was shocked at how cold a man Mike's father really was. She confronted him alone. His father said that he regretted what his actions did to Mike. They made him unpopular and sent him to a dark place. He claimed that one never truly overcomes such a dark place. He told Susan to watch Mike carefully.
While at Bree's house, Mike took some pills and dropped one. Bree found it and researched it. When she found out what it was, she told Susan that Mike was using drugs. Susan didn't believe it at fist but found pills hidden in Mike's flashlight. She confronted Mike and Mike spilled the pills down the drain to convince her that he wasn't an addict. However, that night, Mike undid the pipes and retrieved the pills.
Mike's drug dealer soon began contacting him for money. Mike went to Adam and blackmailed him into getting him more pills. Adam blackmailed Orson into getting him the pills. Mike unintentionally guilted Orson into giving him a prescription.
When Mike's drug dealer showed up at the house, Susan, not knowing who he really was, tried to fix him up with Julie. When Mike found out, he revealed who he was to Susan. Susan threw the drug dealer out of her house. He confessed to Susan that he had bought drugs from the dealer to take away his pain. Susan thought that he had stopped using them but soon learned that Mike was seen picking up a prescription at a pharmacy. Susan later found the pills in Mike's car, and saw that they had been prescribed by Orson Hodge.
In Something's Coming, Susan confronts Mike about the pills; they get into a fight, and Mike accidentally pushes Susan down the stairs. Mike takes Susan to the hospital. Because of his concern about Susan's baby, Mike desperately punches out a hospital staffer, and ends up in a seat in handcuffs. Later Susan informs Mike that the baby is fine and that she suffered a mild sprain. When Susan suggests that Mike go to rehab, Mike refuses. But then she threatens to leave him if he doesn't go to rehab. In the end, Mike agrees to go to rehab. Mike returns from rehab in thirty days' time. Mike doesn't go to the first Lamaze class, as he is receiving his 30 day chip (30 days without drugs). Although he is there for the second.
When Mike tells Susan that Bree Hodge's husband, Orson, was the one who ran Mike over with his car, Susan furiously told Orson not to go near Mike ever again. She also told Bree what happened nearly two years ago. Orson grew apart from Bree because of this, and Susan apologized for it.
After Mike's mother comes to Wisteria Lane for a visit, she makes Susan's life a bit hard, seeing though she wants the perfect housewife for his son. During the part where Mike scolds her mother for this, and right after her mother apologizes, Susan goes in to labor, and gives birth to their baby boy. Mike wanted to name him Maynard after his dead grandfather (who was a father figure for him) but Susan preferred Conner. They stayed with the name Maynard Delfino.