Gardner was elected as a state senator from Cleveland County, North Carolina and served one term as President Pro Tempore of the North Carolina Senate. He served a term as Lieutenant Governor of North Carolina (1917-1921). In 1920, he lost the Democratic nomination for Governor to Cameron Morrison. The election was likely stolen by the political machine of U.S. Sen. Furnifold Simmons, who, through his lieutenant, A. D. Watts, used racial demagoguery and electoral fraud to favor their candidate, Morrison. Gardner led in the initial vote count, but after several days of counting, more votes for Morrison were "found" in western North Carolina, and Morrison won the June primary by 87 votes. Gardner lost the runoff to Morrison by a wider margin.
Gardner made peace with Sen. Simmons. He agreed to support Simmons's favored candidate for Governor in 1924, Angus Wilton McLean, in exchange for Simmons supporting Gardner when he ran for Governor again in 1928. While Gardner was out of politics, he pursued business interests, including a textile mill.
Meanwhile, Gardner took a pro-business, anti-union stance in a period of labor unrest, including the Loray Mill Strike. He did, however, push through the legislature a workman's compensation law and successfully mediated a massive 1932 strike of mill workers in the Greensboro-High Point area.
President Harry S. Truman appointed him Under Secretary of the Treasury (1946-47). In 1947, Gardner was appointed by Truman to be ambassador to the United Kingdom, but he died before ever arriving in London.
Gardner-Webb University is named for Gardner and his wife, Fay Webb Gardner. The fine arts building at Gardner-Webb University is named for him as well. Gardner Hall, a dormitory at Appalachian State University, is also named for him, as is the economics building at the University of North Carolina at Chapel Hill. The O. Max Gardner Award was established in his will to recognize University of North Carolina system faculty who have "made the greatest contributions to the welfare of the human race." It is the only award for which all faculty members of the 16 UNC campuses are eligible and is considered the UNC system's highest faculty honor.