[3][4][5] Cahill started her political career by working as a receptionist and caseworker for Congressman Robert Drinan, a Jesuit priest, after which she became Chief of Staff for Representative Barney Frank.

[7] She was also an Assistant to the President and Director of Public Liaison[8] in Bill Clinton's White House and chief of staff to Senator Ted Kennedy (D-MA).

She also co-founded The Atlas Project, a political consulting firm that worked primarily with Democrats, labor unions, and liberal activist organizations.

Cahill was the Executive Director of EMILY's List for five years and has trained women on how to run for political office in countries including Russia, North Macedonia, and Ireland.

In March 2024, Cahill was announced as co-leader of a new team within the Democratic National Committee to combat third-party and independent challengers in the 2024 United States presidential election.