Ramsden, formerly head of science and education at the channel, will be in charge of features and daytime commissioning, including responsibility for the Big Breakfast's replacement, RI:SE.
The new role makes her the focal point for independent production companies in the restructure, which sees five programme departments out of 13 axed by Thompson in an attempt to simplify the decision-making process.
A spokesman said: 'It was a lengthy commissioning chain, very centralised, and Mark wants to push that down to the editorial teams. It is all about clarifying. He also wants to see a "creative risk element coming back into play.'
Janice Hadlow will head the new specialist factual department, which takes in her existing remit - history, religion and arts - as well as science, education and performance. Although the head of comedy and entertainment, Danielle Lux, loses RI:SE, she will take over commissioning for digital entertainment channel E4.
'We are coming into an increasingly competitive market and in order to compete with bigger players we need to react quickly to changing conditions,' the spokesman added.
Peter Dale, head of documentaries, will also take over multicultural programmes. David Brook, previously director of strategy and development, is now director of marketing and strategy.
Having fewer departments would reduce overlap between departments, but Thompson warned it would also mean job losses in the organisation: 'Creating a leaner, less hierarchical Channel 4 will mean employing fewer people.'
Janey Walker remains managing editor - commissioning, while Tim Gardam is still director of programmes. C4's other departments - news and current affairs, nations and regions, drama and sport - are unchanged.