All R&T employees are now beneficiaries of the Suzanne Richmond & Marjorie Towers Employee Ownership Trust, named in honour of the duo who founded the business in 1930.
R&T, which employs 31 people and generates about £3m annually, was previously owned by Rob Metcalfe, Nikki Thomson and Matt de Leon.
The trio now have different roles, with former CEO Metcalfe becoming chairman, managing director Thomson being made CEO, and deputy MD de Leon transitioning to MD.
R&T said all employees will share dividends - the first will offer £3,600 to each tax free. Employees will also receive a share of the proceeds, based on seniority and length of service, should the company be sold.
The agency also said the structure means "improved engagement from everybody in the company in the way that it is run". It also offers "valuable experience of working in an employee trust-owned business".
Since late 2014, UK firms have been able to create EOTs if they transfer at least 51 per cent of ownership into the trust.
Thomson said: "This is a very exciting day for our business. We think we are the first [PRWeek UK] Top 150 PR agency to be 100 per cent employee owned, reflecting the pioneering attitude that has kept the business successful for so long.
"We believe employee ownership is the right model for a forward-looking company like ours, where so much is dependent on our people as individuals and the way they work together as a team. Rather than working for a small number of shareholders, everyone at R&T is now working for themselves and for each other. We are sure that our clients will see the benefit of this approach, too. All the comments that we have had so far are incredibly positive."
Deb Oxley OBE, CEO of the Employee Ownership Association, said: "We congratulate Richmond & Towers on its transition to employee ownership. By widening ownership of the business to its employees, Richmond & Towers is securing the future of the business, including its ethos, values and employees, while rooting itself locally for the longer term to the benefit of the communities it serves."
Advisors on the new sale included Moore Kingston Smith, Simons Muirhead & Burton, Freeths and Jurit.
R&T, which claims to be the UK's oldest PR shop, has historically specialised in food and drink. Clients added in the past couple of years include McVitie’s, Pukka, Estrella Damm, Westons and Galderma. More recently, it has added clients in FMCG, insurance, retail, leisure, fashion, beauty, energy and the built environment.
This article was updated on Monday afternoon with more information of the employee benefits.