Mike Guthrie, born Richard Michael Guthrie, is a Los Angeles security guard who was working in a hospital when he caught a woman, Sheila Carter, attempting to tamper with a blood test meant to determine the paternity of baby Bridget Forrester, who had potentially been fathered by Sheila's fiancé, Eric Forrester. Mike taunted Sheila and spun the carousel containing the test tubes, rendering the results completely inconclusive. Mike scoffed when Sheila threatened to castrate him with a pair of scissors if he blabbed about her involvement with the test. After Sheila married Eric and took a position at his fashion design house, Forrester Creations, Mike attempted to blackmail Sheila into giving him a job there. Mike met Sheila to accept his new post but found himself attacked by a Doberman that Sheila sicced on him. However, Mike, having had experience training the breed, quickly subdued the dog and turned the tables on Sheila by menacing her with it.

This led to Mike gaining Sheila's respect, and he soon became her friend and confidant. When Sheila had trouble in her marriage to Eric and wanted to get pregnant by him to secure her place, Mike stole a copy of Eric's will at her request so she could see how she fit into things. Mike kept quiet when he learned that Sheila was indirectly responsible for the death of her therapist, Jay Garvin. Later, Mike listened in horror as Sheila revealed that she had stolen a baby from Lauren Fenmore in their hometown, Genoa City, Wisconsin, and that Sheila was presumed dead there after presumably perishing in a fire.

Mike came to Sheila's aid when Sheila had supposedly drowned the visiting Lauren in a hot tub; when Mike arrived, Lauren, who had survived and was ready to expose Sheila's Midwest misdeeds to Sheila's new psychiatrist, James Warwick, had gotten away. Before Lauren could get to James, Mike helped Sheila drug James and imprison him in the basement of Sheila's house, which had once been owned by escape artist Harry Houdini; Mike enjoyed James's torment as Sheila locked James in a variety of devices Houdini had left behind. By this time, Mike had fallen in love with Sheila and couldn't hide his hurt when James picked up on Mike's feelings and declared that Sheila did not return them.

Mike saw through James trying to make Sheila fall in love with James and tried to warn Sheila. When James escaped, Mike was relieved to hear that Sheila had gone on the run, but he received a shock when Sheila showed up at Mike's apartment, begging for Mike to hide her. Mike reluctantly complied and threw the police off the scent as he watched Sheila invite the Forresters to some kind of party and whip up a vial of poison to take with her. The next day, Mike breathed easy when he discovered that Sheila was gone, at least until he found his gun missing. Sheila had taken it and held the Forresters at gunpoint before attempting suicide with the poison, after which she was incarcerated in a prison hospital.

Mike resumed his friendship with Sheila once she was released and stole an invitation to the wedding of Sheila's former best friend, Brooke Logan, so Sheila could find out where the ceremony was. After Brooke called Sheila crazy and demanded that Sheila stay away, an angry Mike pulled a gun on Brooke to force her to let Sheila attend, but he was tackled by his fellow security guards for his efforts and fired from Forrester Creations.

A desperate Mike took a job with Forrester's competitor, Spectra Fashions. On the orders of Spectra's owner, Sally Spectra, Mike arranged to save Eric from a "mugging" and got his security guard position back; inside Forrester once again, Mike took photos of Forrester designs for Sally and turned them over to her so she could copy them. Not receiving payment from the cash-strapped Spectra, Mike quit.

Soon, a letter turned up declaring that Brooke had paid a doctor a million dollars to state that Brooke's husband, Ridge Forrester, was Bridget's father, and not Eric; Sheila became a suspect when the doctor turned up dead, and Mike boasted about how he would have killed the doctor, not knowing that Sheila was recording his "confession." Mike was forced to leave L.A. when Sheila threatened to go to the police with the recording.

Later, Sheila fell in love with James, who had become involved with Maggie Forrester. To pave the way for Sheila, Mike kidnapped Maggie and held her prisoner in an amusement park. Evading the police after Maggie's rescue, Mike was on hand after Sheila had James's baby but agreed to let James and Maggie adopt it. Mike clandestinely took a photo of the infant for Sheila to see. When Sheila reclaimed custody of her little girl, Mary, Mike discovered that the Warwicks were trying to prove Sheila an unfit mother and stepped in to "babysit" Mary when they tricked Sheila into abandoning the child.

After James left Maggie for Sheila, Maggie hired Mike, along with his brother, Martin Guthrie, to kidnap Sheila so Maggie could try to reunite with James. Mike and Martin held Sheila captive in the Psycho house on the Universal Studios lot, where Mike was "attacked" in the shower by the fun-loving, alcoholic Martin in a Ma Bates costume. Mike lost Sheila's friendship over the scheme, but later, when Sheila was remanded to the psychiatric hospital for terrorizing Stephanie Forrester, who had an interest in James, Mike warned Sheila about a hearing that would grant James full custody of Mary and sat in the back of the courtroom in disguise as the case was decided in James's favor.

Mike agreed to help Sheila after she broke out of prison, arranging for a friend of his to alter Sheila's appearance so she could get near Mary. Mike tried to talk Sheila out of her plan to exact vengeance on Stephanie, who had been instrumental in sending Sheila back to prison. As Sheila prepared to leave town, Mike admitted his feelings for her and suggested they could have been good together; Mike was surprised to receive a tender goodbye kiss from Sheila.

Mike disappeared from the canvas until 2010, when it was revealed that he had turned to a life of crime. Mike sold a gun to Brooke's father, Stephen Logan, who was looking to get even with Stephanie for her inadvertent part in the drowning of Stephen's Alzheimer's-afflicted wife, Beth Logan. Mike commiserated with Stephen over their hatred for the Forresters and hinted that Sheila, who was presumed dead at the time, was still alive. Mike got caught when he ran into Brooke and was forced to tell her that her father had been dating Stephanie's sister, Pamela Douglas, who had bipolar disorder, in the hopes of getting Pam off her meds so Pam would shoot Stephanie.

In 2022, Mike, who had been working as a guard in a Los Angeles prison, facilitated the escape of a very much alive Sheila, who was serving time for shooting her son John "Finn" Finnegan and his wife, Steffy Forrester. Mike was dumbstruck when he learned that the comatose Finn was being kept alive in a makeshift hospital room created by Finn's adoptive mother, Li Finnegan. When Li was confronted by the fugitive Sheila and ran away to call the cops, Mike reluctantly lent Sheila his car, which led to Sheila running Li off the road and killing her. Mike, who still had feelings for Sheila, felt Sheila was in over her head and watched as Sheila brought Finn back to life after Finn flatlined. Mike got nervous when he was questioned by the police and urged Sheila to leave town with him. But when Mike walked in and found a not-dead Li accosting Sheila, who was tied to Finn's bed, Mike knocked Li unconscious and helped Sheila flee. Just as Mike was menacing the revived Li, the police burst in, and Mike was arrested.

After Sheila had also been incarcerated, it was said at her hearing that Mike had taken Sheila out of prison by force, which caused the charges against Sheila to be dropped; Sheila was freed while Mike continued serving time. Months later, Mike visited Sheila, who had returned to jail after falling victim to a sting set up by publishing magnate Bill Spencer, Jr. As Sheila had been arrested for a long-ago murder, Mike warned Sheila that it was best for her to accept her imprisoned fate, wary when Sheila confided a new scheme to escape.