The publisher behind Haymarket’s Brand Republic, Conference & Incentive Travel, Campaign, Event, Marketing, Media Week and PR Week, said she is leaving to "try something new".
During her tenure Stilwell has been instrumental in launching a raft of new products and events across the group, and most recently was behind the launch of the inaugural Brand Max event.
Stilwell said: "I have enjoyed every minute at Haymarket and I am honoured to have worked with so many wonderful people and to have been part of the growth of so many amazing brands.
"I will be very sad to leave but I believe that it is time for me to try something new, utilising all the skills I have learnt and the experiences I have had."
Stilwell began her Haymarket career in classified recruitment advertising sales, where she blazed a trail for market-leading brands Campaign and Marketing.
Jane Macken, managing director of Haymarket Business Media, said: "Rachael has made a significant contribution to HBM over the years and we will miss her terrific passion and energy.
"I have no doubt she will be a great success in whatever she does next and I wish her all the very best."
Haymarket is expected to announce details of Stilwell's replacement in the coming days.