Their romantic relationship started in the fourth episode of Season One, but ended in the fifth when Finn's girlfriend came to Earth. However, at the end of season one they grew closer together after Raven ended her relationship with Finn. Their relationship officially ended in the eighth episode of season two, due to Clarke mercy-killing Finn.
In Pilot, Clarke and Finn meet for the first time when they, and the other 98 delinquents, are on the way down to Earth. He decides to take a space walk during the descent, and Clarke establishes that she has heard of his previous misdemeanors, meaning his spacewalking while on the Ark, the reason he was arrested. Though she appears to have a bit of a hostile and guarded attitude toward him during this exchange, when they hit turbulence, she worriedly asks if he's alright. Later, on Earth, he calls her Princess and questions her about the map she is looking at. She tells him that they were dropped in the wrong place. They decide to go on a mission together to Mount Weather to go get supplies. However, they never make it there because Jasper is attacked and kidnapped by a Grounder.
In Earth Skills, Finn helps Clarke search for Jasper, and later helps her and the other delinquents bring him back to camp. He also shares his food with Clarke later that night.
In Earth Kills, Finn helps Clarke look for a medicine for Jasper, and takes Clarke to an abandoned vehicle to hide in during the Acid Fog storm.
In Murphy's Law, he gets Clarke a pencil from the bunker they found. He also helps Clarke hide Charlotte when Murphy tries to kill her, and later witnesses as Clarke banishes Murphy from camp. Once the attempt to contact the Ark fails Finn freaks out and begins to trash the bunker. Clarke manages to calm him down, they kiss, and have sex.
In Twilight's Last Gleaming, Finn and Clarke wake up after having sex and begin kissing in bed again. They later sit together and watch the stars. They share a kiss. When morning comes, Finn goes with Clarke to see what is in the pod that has recently been sent down from the Ark. Finn is surprised to see his girlfriend, Raven. They passionately kiss in front of Clarke, leaving her shocked and hurt. Finn later apologizes to Clarke and helps Raven send up flares into space as a way to contact the Ark in order to prove that Earth is survivable.
In His Sister's Keeper, Raven finds out about Clarke's relationship with Finn and is less than pleased. Finn is stabbed by a Grounder with a knife (Later on, the blade of the knife is revealed to have had poison on it). Clarke is shown to be very upset and rushes to help him.
In Contents Under Pressure, Clarke allows Bellamy to torture the Grounder in order to get him to give up the antidote that will save Finn's life. Clarke manages to save Finn's life after the Grounder eventually gives up the antidote. After saving Finn, Clarke cries into him and says that she can't do this without him.
In Day Trip, Clarke leaves Finn with Raven so she could go gather supplies with Bellamy. Finn is upset that Clarke is letting Bellamy bring guns into the camp and says he wishes Clarke had told him first. However, Clarke says she wishes he'd told her something first as well, in reference to Raven.
In Unity Day, Finn gets Clarke to agree to a peace meeting with the Grounders. He holds her hand briefly on the bridge where the peace meeting is going to take place, which makes Raven jealous. However, it ends badly when Bellamy, Raven, and Jasper begin shooting at the Grounders.
In I Am Become Death, Finn catches Clarke as she falls from illness, which also makes Raven jealous. Raven breaks up with him because she believes that he loves Clarke more than he does her.
In The Calm, Finn and Clarke are kidnapped by Anya and forced to save a young tribeswoman's life. Her name is Tris, and Anya states that if she dies she will also kill Finn. Clarke fails to save Tris and Finn is taken away by fellow Grounders to be killed.
In We Are Grounders (Part 1), it is revealed that Lincoln saved Finn's life and he reunites him with Clarke. The pair escape Anya's unit through the Reaper tunnels. Finn confesses his love for Clarke but she rejects him.
In We Are Grounders (Part 2), both Clarke and Finn fight in the war against the Grounders. Finn fails to make it to the dropship before the rocket fuel explodes, leaving his fate unknown, and Clarke upset.
In The 48, Finn believes that Clarke, as well as the other delinquents, have been abducted by Grounders. Clarke, while at Mount Weather, worriedly asks Jasper if Finn is in Mount Weather with them, but Jasper sadly tells them that he didn't make it, however, Clarke argues with him by saying that they don't know for sure.
In Reapercussions, Finn notices that a one-eyed Grounder is wearing the watch that Clarke's dad gave her as a necklace. He aids the other delinquents in kidnapping and interrogating the Grounder. He takes a dark path as he gets desperate to find Clarke, and he punches the Grounder repeatedly for information and threatens to shoot him. Later on, even though the Grounder gives the delinquents the information they need about Clarke and the other delinquents, Finn kills him in order to end a dispute between Bellamy and Murphy.
In Many Happy Returns, Finn's "dark path" continues as he gets more and more desperate to find Clarke. At one point, he was even willing to let Mel hang off of an edge of a cliff, and most likely die due to her being exhausted and having trouble holding on, in order to find Clarke.
In Human Trials, Finn reaches the breaking point trying to find Clarke. He starts a fire in The Woods Clan's village, takes the members in that area of the camp hostage, and later guns down many of the Grounders. Clarke shows up and witnesses him doing this and is horrified. Finn tells Clarke "I found you" and walks towards her, but she is so terrified of what he had done that she backs away from him.
In Fog of War, Clarke is still scared of the massacre Finn had created. Finn tries to tell Clarke that the massacre was an accident, and tells her that he did what he had to do. Clarke can't even look at Finn for the longest time. Once the pair hide from the acid fog in the bunker, Clarke immediately sees Delano, the Grounder Finn had executed, lying dead on the floor. She is horrified by this and Finn immediately covers up Delano's body. Later, Finn hands Clarke her father's watch. Finn also claims that Clarke is looking at him like he's the enemy, but Clarke disagrees, and Finn points out that she doesn't look at him the way she used to. Clarke admits that she doesn't even know who Finn is anymore, to which Finn replies "Neither do I".
In Long Into an Abyss, Clarke defends Finn when Nyko tries to kill him while they are in the dropship. Later, Finn explains to Clarke that even if she can save Lincoln he will never be the same because of the things that he has done when he was a Reaper. Clarke assures Finn that Lincoln is fixable, and so is he. Finn also encourages Clarke to go make peace with the Grounders. At the end of the episode, Clarke is shocked when Lexa demands Finn in return for a peace treaty.
In Spacewalker, Clarke returns to Camp Jaha with devastating news; the Grounders want Finn in return for a truce. Clarke does everything she can to protect Finn from the Grounders. Finn admits that he is in love with Clarke, and that's why he massacred all those Grounders; he didn't care what happened as long as she was safe. Clarke then begs him to stay instead of running away. Both Clarke and Finn are attacked by a Grounder, however, Finn manages to scare the Grounder away, but Clarke is already lying unconscious on the ground next to him. He carries her to the dropship and stays with her. When she wakes, he admits that he thought she was dead because of him. Clarke tells Finn that the things all of them had to do to survive doesn't define who they really are, but Finn questions this and points out that this could be who they are now. Clarke, Bellamy, Raven, Murphy and Finn come up with a plan to protect him from the Grounders. However, Finn decides to surrender and allows the Grounders to take him away. Later, as he is waiting to be killed, Clarke tries to reason with Lexa by saying she's covered in Grounder blood and that Lexa should kill her instead of Finn. Lexa points out Finn is guilty, and Clarke realizes it's hopeless, so she asks to say goodbye to her lover, and Lexa accepts. Clarke immediately runs to Finn and kisses him and tells him that she loves him, too. They passionately hug and Finn admits that he's scared, but Clarke reassures him that he will be okay. Finn says "Thanks, Princess" and his head drops to Clarke's shoulder. Clarke backs away, holding a knife in her hand which covers both the knife and her hand in blood. Finn has a stab wound in his stomach. Finn said "Thanks, Princess" to thank Clarke for mercy-killing him, stopping his long and painful death that the Grounders were going to give him.
In Remember Me, Clarke shows much sadness after Finn's death. She sees him in various hallucinations. In the last hallucination, Clarke tells him that "Love is weakness" and he expresses sadness, but quietly turns and walks away and eventually fades away.