Harrison worked at Number 10 for two years until July 2019, serving as spokesman for the Prime Minister following the 2017 General Election.
He advised on domestic and foreign affairs and Brexit, and received an OBE in Theresa May's Resignation Honours list.
Previously, Harrison worked for four years as special adviser to the Secretary of State at the Department for Health, and before that as head of public affairs at schools watchdog Ofsted.
His appointment as senior counsel is intended to strengthen Lexington's broad-based political team.
Harrison said: "I am delighted to be joining Lexington at what will clearly be a busy time politically, and I look forward to working with an excellent team."
Ian Kennedy, partner at Lexington Communications, added: "Paul has a unique perspective following a successful career at the centre of government. He will add significant insight to our already outstanding political team."
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