Stock will take the newly created role from January 2015. The company employs more than 5,000 people working across titles such as the Daily Mirror and the Sunday Mail.
She joins from digital publisher Hearst Magazines UK, where she has been HR director since April 2012. She has previously held director-level HR positions at Penguin Random House and the BBC.
Trinity Mirror chief executive Simon Fox said he was delighted to have “attracted someone with Rachel’s calibre”.
“Creating a positive and collaborative working environment for everyone who works for Trinity Mirror is incredibly important to me,” he said. “Rachel’s experience will be invaluable as we build on the important work we have already achieved in transforming our business and culture.
“Ultimately, I want Trinity Mirror to become one of the most open, creative and dynamic places to work and Rachel’s leadership across HR will help us to realise these goals.”
Stock added she is looking forward to working with Fox and the wider team on the “ongoing transformation of the business”.
“Trinity Mirror is an organisation with a great heritage, iconic brands, talented people and it has an exciting and ambitious future. I am thrilled to have the opportunity to be part of its future success,” she said.