Cicero's London team will move into Havas' UK HQ - the "Havas Village" in King's Cross - on Monday. They will sit alongside Maitland, creating a combined workforce of 110 consultants.
AMO, which launched in 2001, is a selection of comms agencies located across the world - many owned by Havas - that agree to collaborate.
Cicero, which will now operate as Cicero/AMO, topped the Public Affairs table in PRWeek's UK Top 150 last year with UK revenue of £5.3m in 2018, following its acquisiton of Westbourne Communications. Revenue for 2019 is set to be about £7.8m, with a 65-strong workforce.
The agency, founded in 2001, is led by executive chairman Iain Anderson and chief executive Jeremy Swan.
Explaining the move to include Cicero in AMO, Maitland/AMO chief executive Neil Bennett told PRWeek: "We've had a small public affairs arm for a while, but I was keen we have access to real scale in that area.
"[Cicero] is London's leader in public affairs and policy and we are one of London's leaders in financial communications and M&A communications, and I could see the whole corporate communciations area getting increasingly political and politicised."
He added: "One of the strategic imperatives for AMO has been building public affairs. Iain and Jeremy will, we hope, be the driving forces in AMO's public affairs global offering."
As evidence of AMO's public affairs expansion, last year AMO agency Abernathy Macgregor opened a government affairs office in Washington. The network also has a public affairs presence in Havas Paris/AMO and Germany's Deekeling Arndt/AMO.
Anderson told PRWeek that joining AMO "allows us to start collaborating immediately with the rest of the Havas world".
"We turned ourselves into Britain's biggest public affairs business on our own. Plugging ourselves into this network, I think, supercharges it even more."
He added that there is "not a lot of overlap" between existing work carried out by Maitland/AMO and Cicero.
The idea was to shift from "a collective of agencies" to a "real network", with a new central management team put in place to spearhead the expansion.
AMO currently operates in 11 countries. In the first half of 2019, the network advised on almost 60 M&A deals, worth more than €230bn.
Its chairman, Angus Maitland, said: "We are delighted to welcome Cicero into AMO’s growing global network and we all look forward to working with Iain and Jeremy. This is part of a broad strategic move to develop AMO capability in public affairs and policy, as governments around the world are becoming more interventionist in corporate affairs, particularly around M&A and governance."