Jon Snow is the claimed bastard son of Eddard Stark by a mother whose identity is not widely known. Jon's direwolf companion is the mute albino he named Ghost.

Jon, who received the surname customarily assigned to illegitimate northerners, is of an age with Robb, Eddard Stark's eldest legitimate son. He was sired during the War of the Usurper, and Ned took possession of him some time after the battle at the tower of joy, which only Ned and Howland Reed survived. Despite his wife Catelyn's objections, Ned insisted on taking responsibility for the boy, and he grew up side-by-side with the trueborn Starks. He has a long face, grey eyes and lean build, with strong Stark coloring—in fact, he looked more northern than any of Lady Stark's trueborn children (except Arya), which was part of the reason Catelyn could never accepted his presence in her household. Unlike the more usual experiences of illegitimate children, Jon lives with his father but has never met his mother, and indeed has no idea who she might be.

As Jon grew up, his position became increasingly awkward. During King Robert Baratheon's visit to Winterfell, his uncle Benjen Stark suggested that the Watch could use a man like Jon. Although Jon's request was initially blocked by Lord Eddard, Eddard's appointment as the King's Hand left him with no option but to accept; Jon would be a serious liability in King's Landing, and Lady Catelyn made it clear that he would not be welcome at Winterfell.

Upon his arrival at the Wall, Jon initially suffered a role reversal: he felt contempt for the lowborn, often criminal members of the Watch, and was resented for being highborn; he was dubbed "Lord Snow" in ironic reference to his upbringing, bastardy and aristocratic attitude. Eventually, however, he put aside his prejudices and assumed a leadership role among them. He united the recruits first against their caustic master-at-arms, Alliser Thorne, and then in protection of the cowardly but good-natured Samwell Tarly. At the outbreak of the War of the Five Kings, he tried to desert the Watch to join Robb's army; however, his new friends found him and brought him back to the Wall before his act was discovered. By finally deciding to honor his vows and abandon his past, Jon came to accept the men of the Night's Watch as his new brothers.

As a fine swordsman, Jon had expected to be made a ranger after his training, and so initially took his appointment to the steward corps as a slight. However, Lord Commander Jeor "Old Bear" Mormont requested Jon to be his personal steward and squire, to groom Jon for future leadership. During a wight attack on Castle Black, Jon saved Mormont's life and received serious burns on his hand. Mormont recognized Jon's courage and loyalty and gave him the Valyrian bastard sword Longclaw. Although it was the hereditary sword of House Mormont, the Lord Commander's own son was in disgrace. The sword's pommel was altered from the Mormont bear into the Stark direwolf.

During the expedition to investigate the disappearances of ranger groups, Jon was assigned to a scout group under the famous Qhorin Halfhand. Along the way, Jon captured a wildling woman, Ygritte, instead of killing her. The party was later discovered and pursued by wildlings. Before they were overtaken, Jon had a dream in which he was inside of Ghost's body and controlling his actions. When he told members of the Watch, they called him a "skinchanger". Soon after, Qhorin commanded Jon to infiltrate the wildlings and to learn what they were seeking and why they were gathering in such large numbers; he then forced Jon to kill him in view of the wildings, to legitimize Jon's request to join them.

Jon joined the wildlings and linked up with the massive wildling army Mance Rayder had drawn together. Jon met Mance and learned his plans to cross the Wall. For a time, Jon became Ygritte's lover and hesitated between betraying her or leaving the Night's Watch. On a mission to scale the Wall and launch a surprise attack on Castle Black from behind, Jon escaped his wildling comrades to rejoin the Watch. There he took command of the skeleton crew left defending the Wall. Despite overwhelming odds, Jon successfully held off the wildlings' assaults until the timely arrival of Stannis Baratheon's army. Among the wildling dead was Jon's lover, Ygritte.

In the aftermath of the battle, Jon's enemies were quick to accuse him of treason; however, his service defending the Wall and spying on Mance earned him popular support. Jon was released and, due to the efforts of Samwell Tarly, "Lord Snow" was voted the 998th Lord Commander of the Night's Watch.

Jon's next challenge was Lord Stannis. Stannis needed a new Lord at Winterfell, one who could challenge the Bastard of Bolton and secure the North; he offered in exchange his royal decree making Jon Snow into the Lord Jon Stark of Winterfell. Jon remembered his oath to the Night's Watch and refused Stannis's offer, while welcoming Stannis's support and coordinating the Wall's defense with him. Knowing that the Red Priest Melisandre wanted to sacrifice someone with royal blood, Jon sent Mance's child away (with Maester Aemon Targaryen).

Jon does not trust Stannis, and is full of hatred for the Lannisters for murdering his father and brother—with the sole exception of the disfigured dwarf, Tyrion, to whom Jon pledged friendship before the war began.

Jon's parentage is a subject of speculation among the series' characters and fans. Eddard would not speak of the matter: when asked point-blank by Catelyn about the rumor that Jon's mother was Ashara Dayne, he would not answer; years later, he claimed to his best friend Robert Baratheon that a lowborn woman named Wylla was Snow's mother. Both theories have received independent corroboration from other characters, but Ashara herself took her own life shortly after Ned's departure, and Wylla's fate has not been established. Thus, Howland Reed is the only character who has been revealed to know Jon's parentage and is still alive enough to speak of it.