Ms Shiels, who will take up the post from 1 October, has been a member of the panel since 2015 and succeeds Craig Errington, group chief executive of Wesleyan.
Mr Randell said: “The Smaller Business Practitioner Panel plays a vital role in ensuring that the voice of smaller firms in the industry is heard in the UK's regulatory system.
“I am sure that Marlene will continue the excellent work of her predecessors and I look forward to working with her in her new role.
“I would also like to thank Craig for his contribution, over several years, to the work of the panel as a member and latterly as chair.”
Ms Shiels said: “The Smaller Business Practitioner Panel plays a very important role in the regulatory environment.
“It represents a significant number of businesses across the country and ensures that their interests are at the forefront of change.
“I am looking forward to continuing to work with the FCA and key stakeholders to ensure smaller businesses are well represented through the Smaller Business Practitioner Panel.”
The FCA Smaller Business Practitioner Panel is an independent statutory body that represents the interests of practitioners of firms of small or medium size within their sector across FCA-regulated activities.
Panel members are recruited to represent both their own sector and the industry as a whole, rather than individual firms.