Monteith is currently comms director of Global Britain, the business campaign for leaving the EU. He will be seconded to Leave.EU, one of the groups hoping to become the official Brexit campaign, for the duration of the referendum campaign and work from its office at Millbank.
He resigned from the Conservative Party's Holyrood group in 2005. He played an active role in all three Scottish referenda in 1979, 1997 and 2015.
Monteith said: "The coming referendum will be the most important decision the British people will face for at least a generation. It will determine how much more prosperous, secure and democratic our country can be and I am delighted to be given this opportunity to help convince people that leaving the EU can deliver a better future in all three respects."
Andy Wigmore, director of comms for Leave.EU said: "I am very pleased to welcome Brian to the team; he has a depth of knowledge and experience across politics and the press that will ensure our media operation is first class. One of his first tasks has been to develop our regional press operation as we support the 300 groups that are now active across the UK and his input has already seen a huge increase in our social media support and local media coverage."
Leave.EU was launched by UKIP-supporting businessman Arron Banks in the summer. The group is working with US public affairs and referendum specialist Goddard Gunster and recently took on the services of Cambridge Analytica, a Washington DC, New York and London-based firm specialising in data and behaviour-driven work on elections.
Separately, Leave.EU has issued a statement following the launch of the Labour Party's pro-EU campaign today, with Banks saying: "It appears to have adopted the same old unsubstantiated Europhile myths. The truth is, we would be safer, more prosperous and we would bring democratic accountability back to our own Parliament if we left the EU.
"The remain campaign fails to mention the fact that our EU membership is the main obstacle stopping the UK from exercising our strength as a powerful, capable and successful nation."