To answer this question, I read through the wikipedia pages for past Secretaries of State since 1945. I was looking for past Secretaries of State who had long careers in private industry, but no diplomatic or academic (law) experience before hand.
Closest: Kenneth Rush
Since 1945, there truly is no precedent for having a business professional without a background in law or international relations as Secretary of State.
The closest example I could find was Kenneth Rush, who was Secretary of State under Richard Nixon. Prior to entering government service, Rush worked for 30 years in the chemical industry, eventually becoming President of Union Carbide (a subsidiary of Dow Chemical). It's not a perfect example, because before entering private industry he was a law professor at Duke.
Nearly every past Secretary of State had past diplomatic experience with the Department of State. It isn't uncommon for a SoS to have decades of experience with the Department prior to becoming Secretary.
I didn't keep numerical counts, but many (perhaps most) also had relevant academic experience. Many were law professors in the past, and since sometime around the 1970s we started seeing political scientists accept the role.
The third most common background was military. I'm uncertain if military careers are distinct from other civil servants, but there were a handful of SoS who had backgrounds as high level members of other departments (Defense, Labor, etc.).
Why this trend?
I recently purchased a copy of Monitoring Governments, which discusses the history of inspector-generals. Although a bit dated, the book discusses the difficulty in hiring inspector-generals for federal departments.
It would be desirable in many cases (like the IGs) to have business people in federal roles. However, as Light's research showed professionals from the private sector quickly become frustrated with the slow pace of government. Another problem is with the different authority structures and accountability concerns that make governments different from firms. Finally, government priorities sometimes change dramatically very quickly, which is disorienting for some business professionals.