Glenn is first seen rescuing Rick from a horde of zombies in the city of Atlanta, where Rick is hiding in a tank. Using a radio to talk to him, Glenn guides Rick to the location of his group, including Andrea. They find themselves trapped in a department store initially, but manage to escape, with Glenn driving a Dodge Challenger to distract the zombies around the city while the rest of the survivors get away in a truck.[7] They meet up with the rest of the survivors at the Atlanta campsite, where Glenn helps distribute supplies and assist others when needed. He later goes with Rick, Dixon, and T-Dog to find Merle in the city and help Rick regain the guns previously dropped near the tank. However, Glenn ends up getting kidnapped by a nursing home crew posing as gang bangers, who try to exchange him for weaponry. Glenn is released, but his group then finds out that the truck which they used to drive to the city is missing, leaving them with no choice but to return to the campsite on foot. Glenn and the rest of his teammates return to the camp just in time to witness a herd of zombies invade and kill many of the survivors.[8] In the wake of the massacre, he assists in dealing with the corpses and insists that the group bury their people instead of burning them as they did the walkers they previously killed. Ultimately, he and the rest of the group leave the campsite and head to the CDC,[9] where, for a brief time, they enjoy the luxuries it offers. They are on the road again, however, following the self-destruction of the CDC building.[10]

In Season 2, Glenn continues to be a valuable asset to the group, helping to repair the RV and scavenging items from abandoned cars on the highway.[11] When Carl gets shot, he and everyone else are ultimately lead to Hershel Greene's farm, where he sparks up a friendship with Hershel's daughter Maggie. They have sex while on a pharmacy run together, and, when Maggie expresses afterward that their fling was a one-time thing, Glenn continually tries to convince her that their romance should continue. Genuine love begins to form between the two, which leads them to consider a serious relationship. This consideration appears to stop, however, when Maggie feels frustrated and betrayed by Glenn when he does not keep his promise that he will not reveal that zombies are in the barn. She forgives him soon afterwards however, and they start to engage in a relationship.[12] Before Glenn and Rick leave for town to find Maggie's father, Maggie tells Glenn she loves him. Glenn leaves before saying it back. In town, Glenn is wracked with embarrassment and guilt after freezing during a shootout with outsiders. He blames the inaction on his deep love for Maggie, and apparently breaks up with her.[13] In the season finale, when the farm is overrun by walkers, Glenn plays a big part in its defense. During the chaos, he convinces Maggie to abandon the farmyard for the meantime so as to avoid any further danger. He later assures her that her family is safe, and seeing the right opportunity, confesses his love for her. The two eventually reunite with the other survivors at the highway. After discovering that Rick kept secret the fact that they are all infected, Glenn is among the survivors who grow wary of Rick. He and Maggie contemplate leaving camp and abandoning Rick's leadership. They, however, dismiss such thoughts after Rick taunts the group with the option of leaving on their own.[14]

Six to seven months afterward, Glenn has become increasingly proficient at fighting zombies. He and Maggie continue to have a strong relationship.[15] While out on a supply run one day, he and Maggie are ambushed and captured by and brought to Woodbury for interrogation. There, Merle brutally beats Glenn while trying to find the whereabouts of their camp. Despite this, as well as being forced to fight a walker while strapped to a chair and having the Governor threaten to shoot Maggie, Glenn remains unbroken. When he is rescued by Rick's group, Michonne takes him back to the car they had arrived in (leaving the prison group's red wagon back in Woodbury). While they remain behind, Rick and Maggie set off smoke grenades, killing several Woodbury residents and rescuing Merle and Daryl in the process. Glenn shows great frustration (at one point bludgeoning a walker pulled from a nearby vehicle) when he finds out that not only is Merle going with them, but Rick had not taken the opportunity to kill the Governor as well (still believing that he raped Maggie when they were held captive). They return to the prison without Merle and Daryl, but Glenn's relationship with Maggie begins to fall apart, despite her claims that the Governor threatened to cut off his hand if she refused to bow down to him. With Rick starting to have breakdowns while meeting Tyreese's group, Glenn decides to impose himself as leader of the group, thinking about killing the Governor when he will not be expecting it. As he leaves on a scouting mission, he misses an attack made on the prison by the Governor, which results in Axel's death and the destruction of the main security gates. Towards the end of Season 3, Glenn begins to let go of some of his anger, and his relationship with Maggie improves to the point where he asks Maggie to marry him. She says yes.

Glenn plays a minimal role in the first half of Season 4. He and Maggie get a pregnancy scare and they promise not to go out on runs. He is later seen shouting about walkers being in a cell block during an attack. When the survivors find out about a possible outbreak, Glenn, along with the other survivors, are quarantined. He falls ill and later recovers after Hershel saves him. When the Governor arrives and attacks the prison, he is rescued by Maggie and gets separated from her once the bus leaves. After the prison attack, Glenn wakes up and finds himself trapped in the prison after leaving the escape bus. He heads to his room and finds the photo he took of Maggie. He grabs his riot gear, assault rifle, and an alcohol bottle from a survivor and heads out. Along the way, he encounters Tara, a survivor of the attacking army. She explains that she was the reason behind this and has no need for living as she watched her sister and niece die. She also tells him about the death of Hershel, which leaves Glenn saddened. Glenn convinces her that they need each other to get out alive. Glenn throws the bottle at a nearby car to distract the zombies and he and Tara escape. Along the road, Tara mentions that The Governor told her that the prison group were bad people and she didn't believe him for one second. She then apologizes for everything. They both get attacked by walkers and Glenn passes out. As Tara kills the last zombie, a military truck pulls up and three people climb out: Abraham Ford, Eugene Porter, and Rosita Espinosa. Abraham convinces Glenn and Tara to accompany them on their trek to Washington D.C. so Eugene can hopefully stop the virus and save the world. When their vehicle is destroyed by Eugene, Glenn takes this as a sign that he should find Maggie and Tara decides to go with them, then Rosita follows and Eugene tells Abraham that they should go along. The group begins to see signs from Maggie that they should go to Sanctuary and when the group comes to a tunnel, Abraham can't bring himself to risk Eugene's life and they part ways, leaving Glenn and Tara to go through the tunnel alone. They come across the dead wandering around an area in the tunnel that has collapsed and while sneaking around them, Tara falls down the debris and is trapped. Glenn refuses to leave her and begins beating off the zombies, when they are rescued by Maggie, Sasha, Bob, Eugene, Abraham and Rosita. Reunited with Maggie, they all decide to continue to Sanctuary where they meet with the refugees. We see in the last episode that they have been imprisoned in a railroad car when Rick and his group are also imprisoned in the same car.