He died after collapsing while playing his “usual Saturday morning round of golf” on 11 September. He was 69.
Mercer retired from the day-to-day management of the business in 2017. His son Neil is now managing director at the Bristol-based company.
Sales director Steve Spencer paid tribute.
He said: “Phil loved his golf, fishing and his family and was fun to be with.
“He was passionate about his business and very aware that he was responsible for his staff. That’s something that he passed down to myself and Neil – that we are responsible for 50-plus staff. In short, a very fair man.”
Mercer is survived by his wife Sue, son Neil, daughter Samantha and four grandchildren.
The funeral will be held at 11:45am on 4 October at Westerleigh Crematorium, Bristol BS37 8QP.
Philtone’s other co-founder, Tony Billing, retired at the beginning of the year.