Lewis was considered by some a despotic leader of the Mine Workers: he expelled his political rivals within the UMWA, such as John Brophy, Alexander Howat and Adolph Germer. Communists in District 26 (Nova Scotia), including Canadian labor legend JB McLachlan, were banned from running for the union executive after a strike in 1923. McLachlan described him as "a traitor to the working class". Lewis nonetheless commanded great loyalty from many of his followers, even those he had exiled in the past.
A powerful speaker and strategist, Lewis used the nation's dependence on coal to increase the wages and improve the safety of miners, even during several severe recessions. He masterminded a five-month strike, ensuring that the increase in wages gained during World War I would not be lost. Lewis challenged Samuel Gompers, who had led the AFL for nearly forty years, for the Presidency of the AFL in 1921. William Green, one of his subordinates within the Mine Workers at the time, nominated him; William Hutcheson, the President of the Carpenters, supported him. Gompers won. Three years later, on Gompers' death, Green succeeded him as AFL President.
In 1924, Lewis framed a plan for a three year contract between the UMWA and the coal operators, providing for a pay rate of $7.50 per day. President Coolidge and then-Secretary of Commerce Herbert Hoover were impressed with the plan and Lewis was actually offered the post of Secretary of Labor in Coolidge's cabinet. Lewis declined, a move he later regretted. Without government support, the contract talks failed and coal operators hired non-union miners . The UMWA treasury was drained, but Lewis was able to maintain the union and his position within it. He was successful in winning the 1925 anthracite miners' strike by his oratorical skills.
Historian C.L. Sulzberger later described the technique in a 1938 book called Sit Down with John L. Lewis, calling it the "Crust of Bread" speech. Operators who opposed a contract were often shamed into agreement by Lewis's accusations. A typical Lewis speech to operators would go, "Gentlemen, I speak to you for the miners' families... The little children are gathered around a bare table without anything to eat. They are not asking for a $100,000 yacht like yours, Mr.______..." (here, he would gesture with his cigar toward an operator), "...or for a Rolls Royce limousine like yours, Mr. _____..." (staring at another operator). They are asking only for a slim crust of bread." . With the full support of the AFL and the UMWA, Franklin D. Roosevelt was nominated and elected President in 1932, and Lewis benefitted from the New Deal programs that followed. Thanks to the Wagner Act of 1935, labor union membership grew rapidly, especially in the UMWA. Lewis and the UMW were major backers of Roosevelt's reelection in 1936, and were firmly committed to the New Deal.
Lewis sent his best organizers into heavy industry in 1935-37, to organize the auto workers, the glass workers, the rubber workers and others. He supported the illegal sit-down strike (but did not use that tactic in the mines). When the AFL balked at organizing unskilled workers, Lewis withdrew his unions and formed a new organization, the CIO. By 1937-40 the CIO was spending as much time fighting the AFL as organizing, with the result that union political power was divided against itself. During the late-1930s struggle over the AFL's refusal to organize mass production workers, Green became the target of some of Lewis' most stinging attacks while Hutcheson was the recipient of a famous punch from Lewis that came to symbolize the dispute between the conservative AFL and the rebellious CIO.
In the Presidential election of 1940, Lewis, heavily dependent on pro-Soviet organizers, rejected Roosevelt and supported Wendell Willkie, a Republican candidate, fearing Roosevelt's intention for American involvement in World War II. After the Japanese attack on Pearl Harbor the miners issued a no-strike pledge "for the duration" in support of the war effort. However, Lewis repeatedly violated the pledge, most notably in 1943 when half a million workers walked off the job. Public opinion was extremely angry and demanded new laws. President Roosevelt, a traditional ally of labor, felt he had no choice but to seize the mines. Even so, some steel mills closed for weeks and power shortages crippled production.
Lewis continued to be as autocratic as ever within the UMWA: until the passage of the Landrum-Griffin Act in 1959, the UMWA had kept a number of its districts in trusteeship for decades, meaning that Lewis appointed union officers who otherwise would have been elected by the membership.
Lewis retired as president of the UMWA in 1960 and was succeeded as president by Thomas Kennedy until his death in, 1963, when he was succeeded by Lewis-anointed successor W. A. Boyle, who was just as dictatorial, but without any of Lewis' leadership skills or vision.
"[An] eloquent spokesman of labor, [Lewis] has given voice to the aspirations of the industrial workers of the country and led the cause of free trade unions within a healthy system of free enterprise."
Lewis retired to his family home, the "Lee-Fendall House", in Alexandria, Virginia, where he had lived since 1937. He lived there until his death on June 11, 1969. He is buried in Oak Ridge Cemetery, Springfield, Illinois.