Three such areas where Mellon's backing created giant enterprises were aluminum, industrial abrasives ("carborundum"), and coke. Mellon financed Charles Martin Hall, whose refinery grew into the Aluminum Company of America. He became the partner of Edward Goodrich Acheson in manufacturing silicon carbide; they founded the Carborundum Company. He created an entire industry through his help to Heinrich Koppers, inventor of coke ovens which transformed industrial waste into usable products such as coal-gas, coal-tar, and sulfur.
Mellon eventually became one of the four richest people in the United States alongside John D. Rockefeller and Henry Ford.
President Harding, in his inaugural address on March 4 1921, called for a prompt and thorough revision of the tax system, an emergency tariff act, readjustment of war taxes, and creation of a federal budget system. These were policies Mellon wholeheartedly subscribed to, and his long experience as a banker qualified him to set about implementing these programs immediately. As a conservative Republican and a financier, Mellon was irritated by the manner in which the government's budget was maintained, with expenses due now and rising rapidly, with income or revenues not keeping pace with those expense increases, and with the lack of savings.
If the rates were set more reasonably, taxpayers would have less incentive to avoid paying. His controversial theory was that by lowering the tax rates across the board, he could increase the overall tax revenue.
Andrew Mellon's plan had four main points:
Mellon believed that the income tax should remain progressive, but with lower rates than those enacted during World War I. He thought that the top income earners would only willingly pay their taxes if rates were 25% or lower. Mellon proposed tax rate cuts, which Congress enacted in the Revenue Acts of 1921, 1924, and 1926. The top marginal tax rate was cut from 73% to 58% in 1922, 50% in 1923, 46% in 1924, 25% in 1925, and 24% in 1929. Rates in lower brackets were also cut substantially, relieving burdens on the middle-class, working-class, and poor households.
By 1926 65% of the income tax revenue came from incomes $300,000 and higher, when five years prior, less than 20% did. During this same period, the overall tax burden on those that earned less than $10,000 dropped from $155 million to $32.5 million.
Mellon also championed preferential treatment for "earned" income relative to "unearned" income. As he argued in his 1924 book, Taxation: The People's Business:
Mellon's policy reduced the public debt (largely inherited from World War I obligations) from almost $26 billion in 1921 to about $16 billion in 1930, but then the Depression caused it to rise again. By 1935, Franklin Roosevelt had gone back to high tax rates and wiped out Andrew Mellon's initiatives. The top tax rate went to 80% by 1935 and the federal government increased excise taxes to make up for the lost revenue.
Mellon became unpopular with the onset of the Great Depression. Many economists today (such as Milton Friedman and Fed Chairman Ben Bernanke, to give two prominent examples) partially attribute the collapse of the American banking industry to the popularity among Federal Reserve leadership of Mellon's infamous "liquidationist" thesis: weeding out "weak" banks was seen as a harsh but necessary prerequisite to the recovery of the banking system. This "weeding out" was accomplished through refusing to lend cash to banks (taking loans and other investments as collateral), and by refusing to put more cash in circulation. He advocated spending cuts to keep the Federal budget balanced, and opposed measures for relief of public suffering. In 1929-31, he spent much of the time overseas, negotiating for repayment of European war debts from World War I. In February 1932, Mellon left the Treasury Department and accepted the post of U.S. Ambassador to the United Kingdom. He served for one year and then retired to private life.
Mellon did not remarry, though in 1923, his former wife married Harvey Arthur Lee, a British-born antiques dealer 14 years her junior,. Two years after the Lees' divorce in 1928, Nora Lee resumed the surname Mellon, at the request of her son, Paul.
In 1913, along with his brother, Richard B. Mellon, he established a memorial to his father, the Mellon Institute of Industrial Research, today a part of Carnegie Mellon University
During his retirement years, as he had done in earlier years, Mellon was an active philanthropist, and gave generously of his private fortune to support art and research causes.
In 1937, he donated his substantial art collection, plus $10 million for construction, to establish the National Gallery of Art on the National Mall in Washington, D.C. The Gallery was authorized in 1937 by Congress.