Barbara Bullock, 66, embezzled teachers' subscriptions to fund extravagances including $100,000 worth of VIP tickets to American football matches, a $6,800 Buccellati silver ice-bucket, a 288-piece Tiffany silverware set, designer handbags, fur coats, haute-couture clothing and $20,000 worth of custom-made wigs.
She has pleaded guilty to conspiracy and mail fraud, but refused a deal under which prosecutors offered a lighter sentence in exchange for information on two co-accused union officials. She will be sentenced in January.
US Labour Secretary Elaine Chao has questioned why the frauds, carried out over seven years, went unnoticed for so long.
"This case involved more than just the ethical failures of a few unscrupulous officials. For abuses this spectacular to go on this long, there had to be an absence of oversight and a lack of concern for the well-being of union members," she said, announcing new regulations that will make unions more financially accountable to their members.
The Washington union's parent organisation, the American Federation of Teachers, uncovered the scandal during an audit last summer, prompting an FBI investigation. The federation has taken over the running of the union and has launched a civil lawsuit to recover the money.
Prosecutor Roscoe Howard said Bullock, who became WTU president in 1994, displayed "nothing short of brazen greed". Items seized from her home included 37 handbags by designers including Chanel and Fendi, 45 pairs of shoes from Bruno Magli and Salvatore Ferragamo, artwork, jewellery, fine bone china, cut glass and crystal tableware.
Bullock now faces a sentence of up to 10 years. However, her lawyers point to the disparity between this and the recent sentence given to Pat Tornillo, a Florida union chief. He embezzled $650,000 (pound;390,000) from his teachers' union, but was sentenced to just 30 months in jail.