Davey will act as a part-time senior adviser to the company and across its UK and international client base.
He joins the existing team of advisers including former home secretary Charles Clarke, former health minister Paul Burstow and Sir Brian Bender, ex-permanent secretary at the Department for Business and others.
Gavin Devine, chief executive of MHP, said: "We are proud that someone with such a vast wealth of top-level government experience has chosen to join MHP."
Davey, a former Lib Dem MP who served for more than three years as secretary of state for energy and climate change during the coalition government, will primarily be working with the MHP team to give high-level advice on sectors including energy, infrastructure and logistics.
He said: "I am delighted to be working with MHP Communications and to be joining its hugely experienced panel of senior advisers. The company already has a strong reputation for client counsel and I hope to build positively on this in my new role."