Hanbury Strategy's founders are former senior Conservative adviser Ameet Gill and Brexit campaigner Paul Stephenson.
The news of Loudon's appointment was confirmed on Hanbury's Twitter feed.
Loudon had joined Theresa May's team when she entered Downing Street in July. As a political appointment, her role officially ended when the General Election campaign began, and she became Theresa May's second key aide to choose against working on her campaign.
Stephenson said Loudon "knows the current government inside out". She will join next month as a partner at the firm. Loudon also previously worked at Vote Leave.
Separately, the body charged with scrutinising post-Westminster roles of former crown servants and politicians yesterday published letters revealing that it had won briefs to provide Brexit-related advice to Barclays Bank and the Red Tape Initiative. Both are likely to run for an initial 12 months, it says.
The Barclays brief includes work "in relation to how the decision to exit the European Union will potentially impact the business", the Advisory Committee on Business Appointment note says. While warning Gill that he should not draw on privileged information from his time in office, it also notes that the Government's position on Brexit has "moved on significantly" since Gill left office 11 months ago.
Work for the non-partisan RTI project will consist of reports and research, rather than external relations work, ACOBA says.
PRWeek listed the agency as one of the start-up agencies to watch in 2017.
#1 PR agency to watch in 2017 - 'an intriguing prospect' https://t.co/9iEYAS1umi— Hanbury Strategy (@HanburyStrategy) January 12, 2017