The character appears in 20 episodes during Seasons 3, 4, and 5, and then three more episodes during Seasons 6 and 7.
As backstory, the character is said to have attended Brown University and Yale Law School (where she befriended Josh Lyman and dated his roommate). She is depicted as a prominent women's rights activist, first appearing in the third season as the Director of the Women's Leadership Coalition, a fictional interest group. The character is also said to have been, at some point prior to her first appearance in the series, Issues Director for the National Organization for Women, Political Director of EMILY's List, and founder of the Democratic Women's Forum.
The flirtation between Amy Gardner and Josh Lyman, and their subsequent relationship, constituted a major story arc in Season 3 of The West Wing.
In the final episode of season 3, Amy challenges Josh, trying to defeat a welfare-reauthorization bill supported by the President. Josh wins this political skirmish, but his success means the end of Amy's Directorship of the Women's Leadership Coalition, and this causes Amy to end her romantic relationship with Josh, although they remain friends. As Amy stated to Josh after some time had passed: "I fought you, I lost, I had a drink, I took a shower. 'Cause that's how it is in the NBA".
Amy returns in season 4 as an advisor to Senator Howard Stackhouse during his campaign for the presidency as an independent. After Stackhouse drops out and endorses President Bartlet, Amy provides unofficial assistance to the President's reelection efforts, working through Josh. She later serves as Chief of Staff to First Lady Abigail Bartlet, giving her opportunity to get back at Josh by outmaneuvering Mr. Lyman, who had gotten used to running circles around the First Lady's incompetent Chief of Staff. She also helps Deputy Communications Director Sam Seaborn fundraise in his failed run for a seat in the House of Representatives.
When Parker became pregnant, Amy was written out of the show in the fifth season episode Constituency of One. After coming into conflict with President Bartlet over funding for domestic violence prevention in an Appropriations bill for the Department of Health and Human Services, she realizes that her methods did not suit a position at the White House--she prefers to pursue her own agenda instead of furthering someone else's political career. She resigns as Chief of Staff to the First Lady and ends her second relationship with Josh.
Amy reappears in the season 6 episode Freedonia to help prepare Congressman Matt Santos for a debate between Democratic candidates for President. Josh (now Santos's campaign manager) fires Amy when he learns that she has been distributing the same policy advice to all the Democratic candidates. Josh later changes his mind, allowing her to remain as a consultant, but the character is not mentioned again in season 6.
In season 7, Amy lobbies president-elect Santos to select a female Congresswoman from Florida as Vice President after the death of Leo McGarry. While Santos is not swayed by her arguments, he offers her the post of Director of Legislative Affairs in his new administration, telling her when she at first refuses "that it is easier to throw rocks at the building from outside, than it is to rebuild it from the inside". Amy subsequently accepts the role, and as the series ends she is firmly established as an important advisor to President-Elect Santos.