The agency has appointed Arnold as a consultant and plans to make further two hires in order to build a three-strong public affairs team.
Arnold, co-founder of the lobbying agency PoliticsDirect, starts immediately in the role of consultant.
Prior to co-founding PoliticsDirect in 1999, Arnold was acting managing director of the political division at corporate and public affairs specialist The Communication Group. He is now chairman of Tweetminster, the Twitter-based sentiment analysis company.
Arnold will report to Lexis corporate head James Thellusson.
Thellusson claimed the new public affairs team was being created in response to demand from existing clients.
He said: ‘Many clients want to understand how legislation and regulation may shape demand for their products or services, and to adapt their corporate affairs strategies as a result. We can now offer that.’
Among the clients that the public affairs offering will initially be looking after are sustainable agriculture online coalition Farmingfirst.org and Premiership Rugby.