Speaking at Dreamforce in San Francisco, Mayer said mobile "caught a lot people by surprise" and Yahoo was previously not clear on how quickly to make the move to mobile.
She said: "We think of ourselves as mobile first – it is clear it is a wave large enough you can ride it to reinvention. The scary part of reinvention is it happens best with platform shift."
Mayer explained that in her fourth day at Yahoo she was told by a mobile engineer there were only three people in the mobile team, but there were around 60 mobile specialists in the other teams.
Mayer explained: "It was sort of everybody’s hobby and nobody’s job […] I guess it was a crisis, it was an immediate priority".
Under Mayer Yahoo has now increased its mobile team to 400 people and has around 400 million monthly users on mobile.
Former Google executive Mayer said Google chairman and "mentor" Eric Schmidt advised her to not focus on the minutiae of the job.
Mayer explained: "Eric Schmidt said good executives confuse themselves when they convince themselves that they do things. You don’t get to code or do the things you would do earlier in your career, your job is to play defence.
"Your job is to say we are going to run in this direction and clear a path and get the obstacles, the process the bureaucracy and naysayers out the way and help people run as fast as they can."
As a result Yahoo has rolled out a programme called PBJ, which stands for process, bureaucracy and jams, and aims to provide transparency and build trust with employees.
Mayer said she was also looking to hire a senior vice president of design to the board, who will report directly into Mayer.
She said: "Some companies aim to connect and some aim to organise. Yahoo is really around entertaining informing and inspiring when you look at those values design is inherent to all of them."