Coleman is the latest in a line of senior Yahoo! executives to leave the company, as it struggles to compete with rival Google.
Decker plans to merge Yahoo!'s sales division with its corporate partnerships and HotJobs, to form a new division. The new business will be known as the global partner solutions division, and will be led by Hilary Schneider.
Schneider joined the search giant less than a year ago from newspaper firm Knight Ridder, to head up what was then another new unit, Marketplaces, which covers its shopping, travel, autos, real estate and employment sites.
Since then she has already added responsibility for the Yahoo! Publisher Network to her remit. The publisher network brings together hundreds of newspapers as part of a consortium for advertising and content distribution.
In her new role Schneider will look at developing Yahoo!'s classified and listings strategy. She has now assumed many of the responsibilities that Decker, who hired her, previously oversaw, before a reorganisation in December, after which Decker went on to replace Terry Semel in June.
Coleman will stay on until February to help in the handover. In an email message to Yahoo! staff, Decker said that she and Coleman had "mutually agreed" that he would leave the company.
His exit follows that of another senior sales executive in June, Wenda Harris Millard, in an earlier part of the reshuffle.
In a memo to staff, Decker said the reorganisation would allow Yahoo! to form partnerships with other website publishers to give it a wider range of advertising inventory to sell to advertisers, ad networks and advertising agencies.
She said: "We will be able to much more quickly identify and secure the ad inventory that best meets our advertisers' objectives."
Last month, Yahoo! founder Jerry Yang, who took over as chief executive, promised a new strategic plan for the company within 100 days.
This came as profits for the second quarter fell to $161m (£78m) for the three months ended in June, from $164m in the same period last year, representing a 2% decline.
Yahoo!'s growth is lower than the advertising industry average for the same period, which has been in double-digits.
Yang said: "I am focused on doing everything we need to do to strengthen our business, capture long-term growth opportunities and create increased value for our shareholders. By sharpening our focus, speeding execution, building our technology and talent, and investing in key growth areas, we can put Yahoo! on a clear path to fulfil its potential as an internet leader."
Investors are watching Yahoo! closely for signs of improvement. Its shares have lost nearly half their value, dropping from a $43 high in January 2006, to $22.55.