Lexa grew up with the Woods Clan and began her warrior training at the age of two. She was born a Nightblood and was, as is tradition, brought to Polis as a child to be groomed as a potential successor to the Commander. She was mentored by Flamekeeper Titus and later served as Anya's Second

When she was 12, the previous Commander died and Lexa was called to attend the Conclave. In doing so, she stopped her training with Anya in order to take on the responsibilities required by her position. Lexa won the Conclave after seven other. Nightbloods were killed and Luna had fled. She was given the Flame and ascended as the new Commander.

At some point, Lexa began a relationship with a Grounder named Costia. The Ice Nation, led by Queen Nia, kidnapped, tortured and beheaded Costia to learn Lexa's secrets. They then delivered Costia's head to Lexa's own bed. Costia's death caused Lexa to trust no one and to believe that love is weakness.

As Commander, Lexa united the 12 Grounder Clans and formed the Coalition, thus becoming the first Commander to rule over all 12 Clans. Despite what the Ice Nation did to Costia, Lexa still allowed them to join her Coalition on the condition that Nia's eldest son Roan was banished.

Lexa's Coalition became involved in a series of conflicts with the Sky People, after the latter group landed in the Coalition's territory. Tensions between the two sides escalated and after the massacre in Tondc, Lexa demanded that the Sky People leave her territory or die. Clarke Griffin instead offered a way to save both of their people imprisoned within the Mountain Men's fortress, Mount Weather, and Lexa agreed to a truce on the condition of Finn Collins's death in compensation for the massacre.

With their new-found alliance, Lexa and Clarke started working closely to free their people from Mount Weather. They grew closer and Lexa admitted to having romantic feelings for Clarke, though Clarke was not yet ready for a relationship. The alliance successfully took down Mount Weather's defenses, but before Lexa's army could attack, Cage Wallace offered Lexa a deal that ensured all of Lexa's people would be freed. In return, Lexa's army would have to retreat and never encroach upon the Mountain again. Lexa took the deal, saving her people but leaving Clarke and the rest of the Sky People to die in Mount Weather. After Clarke was forced to irradiate Mount Weather, the deaths of all the Mountain Men and Clarke's post-traumatic stress causes her to hate Lexa for her betrayal and for endangering Clarke's people.

Months after her betrayal, Lexa found herself struggling to maintain control over the Coalition; her rule being challenged first by Queen Nia of the Ice Nation, and then by Pike's faction of the Sky People. She also had Clarke brought to her in Polis amid the political turmoil. As Lexa worked to bring peace to the clans and earn Clarke’s forgiveness and love.