Johnson succeeds Tim Smith, who is retiring after more than 30 years in the business.
Based in London, Johnson reports to Scot Peachey, CEO, UK construction, infrastructure and surety practice, Marsh JLT Specialty.
Johnson joined JLT in 1988 and has held a variety of senior insurance and risk advisory roles within the Construction Practice.
Most recently, as part of Marsh JLT Specialty, he was head of the global contractors team, where he worked with many national and multinational contractors to design and implement their annual or multi-year insurance programmes.
Peachey said: “With his extensive experience in the national and international construction industry, Mike brings a tremendous amount of knowledge to this role at a time when the sector is navigating its path to recovery from Covid-19.
“On behalf of the business, we also thank Tim for his contribution and dedication over the last three decades, and wish him a happy and fulfilling retirement.”