Sami and Lucas meet under very interesting circumstances, backstage at a rock concert. Austin Reed (Patrick Muldoon), who is on the door, gets Sami inside, so that she can get to meet the singer Cherish. Sami slips into Cherish's dressing room, where Lucas and Cherish have been having sex. Sami gets a good view of a very naked Lucas before Cherish throws them out and tells Lucas to get dressed. The next day Sami goes to Titan Publishing, where Kate Roberts (Deborah Adair) has just finished telling her friend Marlena Evans (Deidre Hall) that Lucas must never find out who his father is. Kate sees Sami, who is Marlena's daughter, and offers her a summer internship. Her first duty is to meet Kate’s imminently arriving son, Lucas.
Lucas does arrive, late, but Sami has been talking to her best friend Jamie Caldwell (Miriam Parrish) on the phone and doesn’t especially care. She does look Lucas over, and feels that she has seen him somewhere before, but now that he is wearing his military academy uniform she cannot place him. However, she says "I know you. You're Lucas Roberts. You're not a goody-goody. You're a bad boy!" The bad boy busily picks the lock of his mother's liquor cabinet. Three days later, she places him, and blushes furiously.
Lucas and Sami soon find themselves attracted to other people. Sami starts crushing on Austin, and Lucas becomes interested in Sami's sister, Carrie (Christie Clark).
Lucas and Sami scheme to break up Austin and Carrie's relationship so they could have them for themselves. Because he agrees to help Sami, she helps Lucas find out who his real father is. His mother Kate Roberts has told Lucas that his father was a successful military man and graduate of West Point Academy but he knows she is lying. In return, Lucas agrees to take Sami to the spring fling wearing his academy uniform and help her make Austin jealous.
Soon, it is revealed that Bill Horton is Lucas's father. Feeling very betrayed and let down, he pushes his mother away. In the process, Lucas gains a lot of family in the form of the Hortons.
Meanwhile, Sami has given up on having Austin and has started dating Alan Harris (Paul Kersey), who went to school with Lucas. Alan is obsessed with Carrie, and after being thwarted in having her, Alan turns his rage on Sami. On a date where Sami came to cook Alan dinner, he pins her down and rapes her, taking her virginity. Lucas notices that Alan and Sami are both behaving strangely, and he corners Sami and asks her what has happened. Sami admits that she has been raped and tells Lucas the whole story. Afterward, Lucas physically confronts Alan and warns him to stay away from Sami. Lucas works hard to get Alan's confession on tape for the rape trial. Some boys taunt Sami about her rape, and in a moment when their guards are down, Lucas and Sami make love - and their son is conceived.
Soon after that, Austin's sister, Billie Reed (Lisa Rinna), is put on trial for the murder of her father Curtis Reed (Nick Benedict). Not only is Billie found to be innocent, but during the proceedings, Lucas's mother, Kate Roberts, finds out that her supposedly dead children by Curtis are alive, and they are Billie and Austin - and so Lucas learns that he has a brother and sister.
When Sami discovers she is pregnant, she claims the baby is Austin's (she had previously drugged Austin, in order that he would have sex with her). Sami believes Austin is the father, and she doesn't even consider the possibility that Lucas is the dad. Sami gives birth to William Robert Reed (Shawn and Taylor Carpenter) in the episode airing November 16, 1995. Lucas becomes very attached to his "nephew" Will, much to the dismay of Sami, who, when Will has to be taken to hospital after a near-drowning, discovers that Lucas is in fact Will's father. Austin and Sami are set to be married, and Sami tries to keep Will's real paternity a secret, but Carrie discovers it, and the secret is revealed to all on Austin and Sami's wedding day, and Austin leaves Sami at the altar.
Lucas wants custody of his son. However, Sami gets in his way, claiming that Lucas has abused Will, and that she has witnessed it. Sami gets a restraining order against Lucas.
Sami befriends Franco Kelly (Victor Alfieri), and they become engaged; Franco uses her to get a greencard and stay in the country, but he is murdered on their wedding day. Sami sees the body and faints near it. Lucas's mother Kate (now played by Lauren Koslow) frames Sami for the murder so that she will go to jail and Lucas can have full custody of Will. But Sami gives custody of Will to Austin when she is sent to jail for Franco's murder. Sami is convicted and is sentenced to death by lethal injection, although she continues to proclaim her innocence. In reality, Lucas and Kate are behind Franco's death. In a flash of drunken rage, Lucas kidnaps his son, and then gets into a car accident, which requires Will to have brain surgery. Lucas has fallen in love with Nicole Walker (Arianne Zucker), but she is in love with Sami's brother Eric Brady (Jensen Ackles), and Kate pays Nicole five million dollars for her to marry Lucas, so that he can gain custody of Will whilst Sami is in jail. Lucas's conscience then gets the better of him, and he rushes to stop Sami's execution, and confesses to the murder in an attempt to save her life. The execution is halted, on the order of the governor - but this is in response to someone else signing a deathbed confession to Franco's murder.
Sami is freed and falls in love with Brandon Walker (Matt Cedeño), who helps her regain full custody of Will from Lucas. They obtain proof that it was Lucas who actually killed Franco, and Sami uses that information to blackmail Lucas – but it is Kate who forces Lucas to give up custody of his son. Losing Will absolutely destroys Lucas, and throws him back into the bottle. While drunk one night, Lucas is caught in the fire that Kate has set to kill Victor Kiriakis (John Aniston), and almost dies. After spending months in a special hospital and then in rehabilitation, Lucas comes back to Salem, more determined than ever to hurt Sami.
Lucas soon gets a job working with Tony DiMera (Thaao Penghlis), and when he finds proof that Sami was involved in criminal activity – switching the paternity results on Lexie Carver's (Renée Jones) baby – he uses this information to break up Brandon and Sami only moments after they are married. Their marriage is annulled. Lucas and Tony are celebrating at the DiMera mansion when Sami bursts in, and in the ensuing scuffle, Sami ends up going through the glass doors. She is taken to the hospital in critical condition, and Lucas goes to a bar to drown his sorrows, and allows himself for the first time in years to remember the friendship he and Sami once shared as he waits to hear if she is going to live or die.
Lucas takes care of Sami after she was released, often against her wishes. As they spend more time together, Lucas finds his heart softening towards his onetime-worst-enemy, and the two share a few nights together (all platonic, of course), and some steamy kisses. When Sami's father Roman (Josh Taylor) is murdered, Lucas is there to comfort her. He is careful though, because he does not want –Will to get his hopes up, and he still hasn't completely forgiven Sami. Lucas and Sami continue to grow closer during the Salem Serial Killer storyline, and ultimately they confess their love for each other. They make love for the first time since Will's conception on April 3, 2004. Lucas successfully proposes to Sami on August 5, 2004 after singing "When a man loves a woman" to her in front of an audience at the Penthouse Grill.
Kate is not pleased, and the night before their wedding, with the help of Eugenia, she drugs Sami and Brandon, and places them in bed together, where Lucas finds them. Lucas leaves Sami, and both are devastated. Sami's sorely compromised psychological state deteriorates to such a degree that she becomes a secret transvestite under an assumed name, "Stan" (played by Dan Wells) – a character that wreaks havoc on Salem for months, working for Tony DiMera, and selling illicit drugs to a pain-wracked John Black (Drake Hogestyn) – all out of a base-born, suppositious need for revenge.
Sami attempts to redeem herself, and with Nicole's help, Sami is able to secure the evidence that Kate and Eugenia had set her up, and she convinces Lucas that she was never unfaithful. The two reunite and agree to marry once again. Moments before Lucas and Sami are to wed, Kate crashes the wedding, and reveals to all that the havoc-wreaking Stan was Sami; and Lucas leaves Sami again. Sami is shunned by everyone in Salem, except for Austin, who had returned for the wedding. She lets Austin move into her apartment. In the meantime, Lucas pursues Carrie, who has also returned to Salem.
Lucas begins to date Carrie, and Sami begins to date Austin, and this leads to the eventual marriage of Lucas and Carrie, and the near marriage of Austin and Sami. But a mysterious gloved hand warns Sami not to go through with her wedding to Austin, or it will be revealed that Sami actually blackmailed Lexie into telling Carrie that she could not bear children.
Carrie cheats on Lucas almost instantaneously, and after Will goes missing, Lucas and Carrie split up, allowing Lucas and Sami to become closer again. Happily Will turns up - he had just run away, because he was embarrassed by Sami jilting Austin at the altar. But Sami's blackmailing of Lexie is revealed anyway, and everyone shuns Sami again, and Lucas and Will move out, leaving her devastated. Austin and Carrie get back together and relocate to Switzerland.
Sami is attracted to her new neighbor, EJ Wells (James Scott), and after her wedding disaster, they kiss for the first time. But Sami then discovers that not only is EJ the son of the villainous Stefano DiMera (Joseph Mascolo), but that he is also sleeping with Kate.
Sami and Lucas reunite, and after getting caught in a snowstorm, Sami and Lucas make love. The roof of the cabin in which they've found sanctuary collapses on Lucas, trapping him. Sami goes to get help, and she flags down a motorist which turns out to be EJ DiMera. She tries to get out of the car but he would not allow her, holds a gun to her, and demands that she help get him through a roadblock as she pleads for him to help her save Lucas. After helping EJ get through the roadblock, EJ tells Sami he will only save Lucas if she has sex with him. She is disgusted and immediately refers to his proposal as blackmail and rape, but she agreed to his terms because she feels it is her only chance to save Lucas. EJ saves Lucas then runs off to Mexico.
Sami is expecting, and on May 9, 2007, to the delight of their son Will (Christopher Gerse), Lucas and Sami are married. At the reception a photo is revealed of Sami and EJ in Lexie Carver's car the night John Black was shot and Lucas was trapped in the cave in. Sami is forced to tell everyone the truth. She is afraid that after the reveal, she will lose Lucas again, this time maybe for good. She is surprised when he refuses to let it ruin the happiest day of their lives. Later at their apartment he tells her that there is no need to confess to him, because he trusts her.
On May 10, 2007, Sami tells Lucas all about the night of the accident and about agreeing to have sex with EJ. Lucas is enraged, immediately calling what EJ did to Sami "rape", and wants to go after EJ. Sami tells him if he does, then the DiMeras would come after them. Lucas is completely understanding when Sami tells him that she only gave in to EJ to save Lucas's life. Lucas tells Sami that he is ashamed of the man he used to be, the one that had loved her and left her in the past, and because of that he thinks that Sami was afraid to come and tell him when EJ had started threatening her. Sami tells him that he isn't to blame, and that she is the one who has messed up time and time again.
On May 22, 2007, the psychic Celeste Perrault (Tanya Boyd), predicts that EJ DiMera is the father of Sami's baby. However, on June 18, 2007 Sami finds out that she is actually pregnant with twins. Sami has a paternity test, but thanks to Kate's blackmailing of Nick Fallon (Blake Berris) to create a false paternity report, EJ is believed to be the father of her twins for a number of weeks. Then, in the episodes airing July 30 and 31, it is revealed that Lucas is apparently the father of both children.
On October 23, 2007 Sami gives birth to a girl and a boy – Allie Horton and Johnny. And in another twist, there is another DNA test, and on November 2, Lucas learns that Johnny is really EJ's son.
Stefano promises to end the Brady/Dimera blood feud if Sami marries his son EJ; and in order to save lives and protect her family, Sami asks Lucas for a divorce. Lucas reluctantly agrees, and Sami and EJ are married in November 2007. Just after the ceremony is completed, EJ is shot in the back and temporarily paralyzed (an affliction he subsequently faked). Lucas sends Will to live in Switzerland with his Austin and Carrie, in order to protect him. Sami and Lucas remained sexually intimate despite her nominal marriage to EJ.
On December 20, Lucas tells Kate that he had hired a nurse to watch Johnny and Allie, gotten his gun, and went to the church and shot EJ. He confesses to the shooting and is sent to prison. Lucas tells Sami to get on with her life. He requests only that she stay away from EJ because he does not trust him.
E.J. tries to take custody of Johnny, but as immigration wants to deport EJ, Sami moves into the DiMera mansion with Johnny and Allie, in order to prove that they are happily married couple. They later had sex, and Lucas, released on house arrest, sees them. Sami ends her marriage to E.J., but Lucas can't forgive her.
In March 2009 Lucas marries Chloe Lane (Nadia Bjorlin), though that relationship ends when she has an affair with Daniel Jonas (Shawn Christian). Sami begins seeing her former FBI bodyguard from the Witness Protection Program, Rafe Hernandez (Galen Gering), becomes involved in a love triangle with him and EJ. Sami and Lucas are good friends and amicably co-parent son Will (Dylan Patton) and daughter Allie. Lucas eventually gets a job offer in Hong Kong and moves there, though he does return for a few days for the funeral of his grandmother Alice Horton in June 2010.
On February 29, 2012, Sami calls Lucas for advice. On March 2, Lucas returns to Salem and goes to see Sami at her apartment. They admitted that they still love each still and they share a hug. Lucas later reveals to Sami that he has a fiancée named Autumn, who Sami calls "Winter" and "April". Once, Lucas found out that Sami was the mole at Countess Wilhelmina, he promised to keep her secret. Stefano DiMera gives Sami the title of CEO at Countess Wilhelmina as revenge for his wife's Kate's affair with Ian McAllister.
On April 23, Autumn breaks up with Lucas, via phone, saying she was sick of waiting on him to come back to Hong Kong.
On April 27, Lucas, Sami, Will (Chandler Massey), and Allie have a fun day. Sami thanks him for everything and gives him a hug. Lucas kisses her on the cheek and they kiss and Kate sees it.
On April 30, Kate and Sami fight and after the fight, Sami learns that Lucas has changed a lot and has grown a backbone. He tells her to follow rules and he will watch her like a hawk. Sami agrees to these terms. Lucas later asks what the kiss was about and Sami kisses him again.
On May 1, EJ sees them kiss the 2nd time and gets jealous and talks about the kids and how if Lucas is in their life they would get confused then; EJ talks about John. They have problems with Countess Wilhelmina but they get them fixed.
On May 2, Sami and Lucas talk about their relationship, and Lucas said he thought about it once but then he had the fiancée now he doesn't they make out until Kate Roberts, Lucas's mother, catches them and talks about CW and how it is her company and then Lucas says it is (Stefano Dimera)'s company. They kiss again.
On August 17, Lucas tells Kate that he is done with Sami for good, which makes Kate happy.
On August 27, Lucas tells Sami officially that they are through. He refuses to listen to her especially after she runs away with EJ. They decided to stay co-parents to Allie and Will.
On May 21, 2013, Sami and Lucas became grandparents to Arianna, Will's daughter. On April 3, 2014, they are proud parents at the wedding of their son Will (Guy Wilson), to Sonny Kiriakis (Freddie Smith).
In October 2014, EJ is shot dead, and Lucas comforts Sami. In order to get the children away from Stefano, Sami relocates to California with Alllie, Johnny, and Sydney (and Will and Arianna accompany them for the first few weeks).
On October 9, 2015, Lucas and Sami's son, Will, is found dead in his apartment. He has been murdered. Sami returns to Salem for Will's funeral. Sami and Lucas go to the morgue together, and grieve over their son's body.