The agency, which is six years old, works with actors, musicians and artists, creating campaigns spanning human rights, the environment, digital rights, immigration, the criminal justice system and healthcare issues.
She has also worked with Russell Brand on a documentary currently in post-production, about death-row inmates and mass incarceration at a prison in Louisiana.
FitzGibbon currently has offices in Washington DC, New York City and San Francisco.
Trevor FitzGibbon, founder and president of the firm, said: "Suzie’s extensive work in the UK, South America, the Middle East and around the world, will be a great asset as FitzGibbon Media continues to help the most important progressive campaigns of our day to break out and break through with audiences across the globe."
Gilbert added: "In today’s interconnected society, it is more important than ever to connect the values of our global community to the change we seek to protect our environment, advance workers rights and ensure that our fundamental freedoms are preserved. We are excited to lend a hand to help push these efforts to a new level."
Last year, the firm partnered with the film-makers of two Oscar-nominated documentaries, The Invisible War and Dirty Wars, developing campaigns and digital engagement strategies.
FitzGibbon also recruited a host of celebrities including Moby, Russell Brand and Susan Sarandon to promote CitizenFour, a documentary providing a first-hand account of Edward Snowden’s disclosure of the National Security Agency’s mass surveillance programme.