Yahoo! was subject to speculation regarding a Microsoft takeover in May, and has endured mixed fortunes over the last year. Semel has signalled that Yang's appointment will kick start a new era of leadership, but has warned of the major challenges it faces.
In November a leaked memo from Brad Garlinghouse, senior vice-president at Yahoo!, said the company needed to cut its staff by 20% in order to redefine its strategy against Google, and that "it's not clear if anyone is in charge".
The memo, which was dubbed 'The Peanut Butter Manifesto', said: "We lack a focused, cohesive vision for our company. We want to be everything to everyone." It went on to say "heads must roll" and that "change was needed soon".
More recently, Semel has overseen Yahoo! missing out on the acquisition of Doubleclick to Google, which is now more than 20% ahead of Yahoo! in the US search market. It is also faltering in the race to buy social networking success Facebook.
Despite Semel tripling Yahoo's wealth during his time as chief executive officer, Google's lead over Yahoo is now stark -- its key rival now makes more money in a quarter than Yahoo! does in a year.
Yang was chosen by the board to succeed Semel. Sue Decker, former executive vice-president and head of the advertiser and publisher group, has also been promoted, to president of the company.
Semel said: "I was clear in telling the board of my desire to take a step back sooner rather than later. I believe Jerry and Sue, with their superb talents and intense dedication to Yahoo! and its people, are the perfect combination to carry us forward.
"This is the time for new executive leadership, with different skills and strengths, to step in and drive the company to realise its full potential -- it is the right thing to do, and the right time is now."
Yang added: "We are well aware of the challenges facing Yahoo! We need to execute better and to get better talent. I feel Yahoo! needed someone to be here for the long haul."