Mitchell joined the agency, which has offices in Westminster, Bristol, Warwick, Cardiff and Manchester, last month, but the announcement was only made today.
JBP was the 85th largest firm in the PRWeek UK Top 150 for 2016, with revenue up nearly a quarter on the previous year to £2.8m.
Mitchell will work four days a week as director of client and media services, a board role that will include national media relations and reputation management, and creating a new media training unit.
Between 2007 and February 2015, Mitchell worked for the agency Burson-Marsteller, before stepping down for an unsuccessful bid to be elected as a Conservative MP in Brighton at last year's general election.
Prior to working in the PR agency world with Burson, Lewis and Freuds, he was the Foreign Office media liaison officer for the McCann parents in the months immediately following their daughter’s disappearance, which later led to his appointment as the family's spokesman in 2007.
Before his PR career he spent 25 years as a journalist and broadcaster, including on-screen roles such as royal correspondent at the BBC.
He said: "I am thrilled to be joining JBP at a time when it has ambitious plans and a growing client portfolio. I look forward to helping JBP add value to its communications proposition and offer its clients the highest level of advice."
Clarence Mitchell Communications will continue to trade, including occasional work for the McCanns.
JBP clients have included the National Grid, West of England Academic Health Science Network and global education firm idoodlelearning.