Dolan, a serial entrepreneur who made his fortune through his accountancy firm SJD Accountancy, has become a partner in the business and a non-executive director.
He lives in Monaco and is involved in a host of companies, from motor racing and engineering firm Jota Sports to board games business Destination Games.
The company declined to reveal the size of the stake Dolan has taken and what he has paid for it.
It said Dolan would act as a consultant, focusing on the strategic direction and growth of PHA, and attend senior management meetings.
He is the first person to buy into PHA Media since it was founded by Hall and his fellow shareholders, managing director Mark Gregory and financial director Marina Hall.
Hall, founder and chairman of PHA, said: "Simon is a friend, a former client and someone all the partners admire and respect. We are looking forward to working with him. He has huge experience of running successful businesses and we believe he will make an enormous contribution in helping drive the strategic direction of the firm."
Commenting on his investment in PHA Media, Dolan said: "PHA Media were my PR advisers for a number of years and I have always admired their business, which like my own company is built on delivering what they promise. I am delighted to be on board and look forward to helping drive this new phase in the development of PHA Media."
Hall said: "We have been seeking to appoint a non-exec director to bring additional experience to the boardroom as we grow. When we mentioned it to Simon he said he would love to get involved but more than that he would like to have a stake in the business because he can see the potential for growth. There's no better endorsement of a business than when one of our clients of many years sees so much potential in PHA that he wants to become a partner."
PHA employs around 60 staff at its office in Wardour Street. Its diverse portfolio of clients includes blue-chip companies from the financial, business, fashion, health and sports markets.