Donnelly stood down as P&G’s brand director for Northern Europe in July after a 31-year career with the FMCG company.
Just Eat is the second non-executive director role she has taken on since then, following Bourne Leisure, owner of Butlin’s resorts.
The brand's chairman John Hughes said: "I am delighted to welcome Roisin to the board of Just Eat.
"Her track record of success in building brands, coupled with extensive international marketing and digital experience, will be invaluable to Just Eat as we continue to develop our offering to consumers and restaurants and grow our industry-leading business."
Just Eat relaunched its brand last month with a new, more colourful visual identity and an updated advertising platform, following the appointment of Karmarama to its creative account earlier in the year.