The new firm will operate under the banner MHP Communications and will boast 160 employees and £17m of annual revenues, transforming it into a top ten player in the UK market.
The integration will be effective from 1 October, with Mandate chief executive Sacha Deshmukh taking the CEO role at MHP.
Gay Collins, co-founder and chief executive of Penrose Financial, will become MHP's executive chairman, while financial PR firm Hogarth's chief exec, Andrew Jaques, will become head of the financial and investor comms team at MHP.
Deshmukh said the new structure was designed to help it better respond to clients' needs. He commented: 'Structuring the business around client sector, rather than PR discipline silo, means we can ensure we are delivering whichever mix of experts and skills are right for any individual client.'
He added the businesses have been brought together without any significant overlap and headcount will continue to rise before year-end.
MHP will move into new London offices in November and has pledged to expand internationally.
As part of its launch, the agency will open a Brussels office that will be fully operational from January 2011 and plans to expand into key markets in Asia and beyond next year.
'We are going to have a genuine global footprint and put together the right mixture of own offices and partners managed by those offices to deliver what is right for those regions,' said Deshmukh.
The combination of the three agencies gives MHP a footprint in consumer, corporate, public affairs, financial services and mid-cap financial and investor relations. As well as expanding globally, it is thought the agency is likely to grow its financial PR offering and extend its focus to larger-cap clients.
MHP also announced the appointment of Paul Baverstock as an MD and head of MHP's corporate reputation discipline.
He joins from Bell Pottinger Sans Frontieres and was previously director of strategic comms at the Conservative Party.
Additionally, Sir Brian Bender will join MHP as a non-executive director. Until 2009, he was the permanent secretary of the UK Department for Business, Innovation & Skills.