He completed his doctoral dissertation, titled "Essays in the economics of uncertainty", under the supervision of Michael Rothschild.

His research centers on the roles that ownership structure and contractual arrangements play in the governance and boundaries of corporations.

His research has emphasized the importance of contractual incompleteness — the inability of parties to contract on every contingency.

[14] He has used his theoretical work on firms in two legal cases as a government expert (Black and Decker v. U.S.A. and WFC Holdings Corp. (Wells Fargo) v. U.S.A.) where companies claimed tax-related benefits as a result from selling some of their business.

[9][11] In June 2024, 16 Nobel Prize in Economics laureates, including Hart, signed an open letter arguing that Donald Trump’s fiscal and trade policies coupled with efforts to limit the Federal Reserve's independence would reignite inflation in the United States.