Harrison-Church, who left her role as Sainsbury's director of brand communications earlier this year, has joined the food company with immediate effect, in the role of director of strategic marketing.
Her focus will be on the optimisation of marketing investment, media effectiveness, consumer PR and developing other areas of marketing capability.
Warren-Piper, currently business transformation director at United Biscuits following time as marketing director of its UK snacks business, will join on 23 October as business unit director, savoury brands.
This will put her in charge of the marketing teams for Bisto, Batchelors, Loyd Grossman, Oxo and Sharwoods, and she will be responsible for driving the brands' growth.
Both will report to Iwan Williams, managing director of Premier Foods.
Premier Foods restructured its marketing department in February in the wake of a profit warning and the arrival of new chief executive Michael Clarke in August 2011.
Clarke’s strategy has been to sell off non-core brands to reduce the company’s debt and focus on eight key ‘power brands’: Hovis, Ambrosia, Mr Kipling, Sharwood's, Loyd Grossman, Bisto, Oxo and Batchelors.
Group marketing director Jon Goldstone left in the restructure and his responsibities were at the time assumed by Williams.
In March the company consolidated advertising arrangements for the power brands into JWT London and McCann London, with Dare and 101 losing out.