Goat joined Livity in 2012 as client services director, and became MD in September 2015. She was previously a board director at Iris London.
She takes over from joint-CEOs, Sam Conniff and Michelle Morgan, who founded Livity in 2001. Both Conniff and Morgan will remain involved, Conniff in the role of chief purpose officer.
As well as providing more conventional creative services to brands, Livity works with a network of more than 4000 young creative people. Its clients include Google, Dyson, Playstation and Barclays.
Conniff said: "Alex is a leader who well deserves the unanimous belief, love and respect she commands across our industry. Her vision and drive for the positive evolution of the agency model is a force that Livity is lucky to have in the driving seat."
Goat said: "As a youth-led creative network, we work extremely hard to surround ourselves not only with the most innovative and diverse talent in the industry, but with young people who inspire, challenge and collaborate daily with us and our clients.
"With a totally unique understanding of young people and their attitudes and behaviours, we take business issues of relevance and purpose for brands' future consumers in our stride, and thrive even more, finding actual solutions to societal challenges - ranging from electoral reform through to sexual consent, to counter-extremism."