St Helens Council, which was originally sceptical about the deal, joined Liverpool, Knowsley, Sefton, Wirral, and Halton in Cheshire in ratifying the plan at a meeting on Thursday.
The deal secures nearly Â£1bn of extra funding over the next 30 years.
Like Greater Manchester, Sheffield, the North East and Tees Valley, it will have an elected mayor.
Former devolution sceptic Councillor Barrie Grunewald, leader of Labour-controlled St Helens, called for an interim mayor before voters elect a mayor in 2017.
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Mr Grunewald said council leaders had not discussed the option yet but added: "My personal view is that we do need to move to an interim mayor model.
"We've signed the deal, the hard work has been done, let's implement it, let's get capacity into the system and let's bring that economic regeneration to all of the city region," he said.
Liverpool City Region's six councils have unanimously backed a devolution plan to transfer powers from Whitehall.