Theodore was born in Adelaide, South Australia, the second son of Basil Teodorescu, a Romanian immigrant. He was educated at Lefevre Peninsula Catholic and Aldgate State schools in Adelaide, but left school at 12 to work on the Adelaide docks. In 1900 he left for the goldfields of Western Australia, but failed to make his fortune there and decided to try his luck at Broken Hill, New South Wales, instead. In 1906 he left for Cairns, Queensland where he prospected for tin in the Chillagoe area and worked in the Vulcan Mine in Irvinebank. It was among the unorganised workers of Stannary Hills and Irvinebank that the Amalgamated Workers' Association of North Queensland was born.
Theodore founded the Amalgamated Workers' Association with Bill McCormack. This union used the process and principle of amalgamation to unify with other unions until it became Australia's largest union, the Australian Workers' Union (AWU). Theodore became Queensland state president of the AWU in 1913. Meanwhile, he had been elected to the Legislative Assembly of Queensland in Woothakata from 1909 to 1912 for the Australian Labor Party and subsequently from 1912 to 1925 in Chillagoe (the seat of Woothakata renamed). His position in the AWU made him a power in the Parliamentary Labor Party, and when Labor won a majority in the Assembly for the first time in 1915, he became Treasurer and Secretary for Public Works in the government of Thomas Ryan.
In 1919 Ryan resigned and Theodore succeeded him as Premier of what was then Australia's only Labor state government, following the great split in the Labor Party over the issue of conscription in World War I. He was a popular and successful Premier, and soon began to be talked about as a possible federal Labor leader. Important educational measures were implemented, which benefited the handicapped and isolated, adult franchise in local government was introduced, and the upper house of Parliament abolished. Workers benefited from the introduction of a compulsory unemployment scheme, changes in the Safety and Accommodation Acts, and the Profiteering Prevention Act. Improvements in housing were also secured through amendments to the Savings Bank Act, the Workers' Homes Act, and the Fair Rents Act. Agriculture also received much attention through measures like the Agricultural Education Act, the Irrigation Act, the Main Roads Act, and the establishment of an Agricultural Bank, a cold store, a state cannery, a Cheese Pool, and a wheat board. Orderly marketing and controls on price fluctuations were also carried out to develop agriculture, while the Theodore Government also involved itself in the marketing of Queensland fruit produce.
In 1925, Theodore resigned as Premier and stood for the Queensland seat of Herbert in the federal election, but was unexpectedly defeated by Lewis Nott by 268 votes.
Theodore was elected to the House of Representatives for the seat of Dalley in Sydney at a by-election in 1927. His status as an outsider in Sydney Labor politics was a permanent problem for him, but he soon made his mark in federal Parliament. In 1929 he became Deputy Leader of the Labor Party under James Scullin. In October 1929 Scullin defeated the conservative government of Stanley Bruce and became Prime Minister, while Theodore became Treasurer.
Two days after the Scullin government was sworn in, the US stock market crashed. The effects of the Great Depression were soon felt in Australia, and the Scullin government, like others, was hard pressed to deal with mounting unemployment and the collapse of world trade, on which Australia's export-based economy depended. Theodore, an early advocate of Keynesian economics, favoured deficit spending to rejuvenate the economy. However, Works and Railways Minister Joseph Lyons and Trade Minister James Fenton supported a more traditional, deflationary approach.
Meanwhile, a conservative government had taken power in Queensland, and appointed a Royal Commission to investigate Theodore's financial dealings as Premier. The Commission found that Theodore and another former Queensland Premier William McCormack, had corruptly profited by authorising the purchase by the state of a copper mine at Mungana while concealing the fact that they had a financial interest in the mine, which furthermore was not economically viable. In June 1930 the "Mungana affair" forced Theodore's resignation.
Without Theodore's leadership and financial skills, the Scullin government drifted into deeper crisis. When it became apparent that the Queensland government did not intend charging Theodore with any offence, Scullin re-appointed him as Treasurer, in January 1931. Lyons and Fenton felt that Scullin should have waited until Theodore had been formally cleared, and resigned from cabinet in protest. Two months later, they and three of their supporters crossed the floor to the non-Labor opposition, and eventually joined forces with the Nationalists to form the United Australia Party, led by Lyons.
During 1931 Theodore faced the greatest economic crisis in Australian history. The government imported an advisor from the Bank of England, Dr Otto Niemeyer, who recommended an "orthodox" solution, including sharp reductions in government spending such as pensions and unemployment benefits. The radical Premier of New South Wales, Jack Lang, on the other hand, campaigned for the repudiation of Australia's debt to bond-holders in London.
Theodore rejected both these alternatives and proposed instead an expansion of credit to farmers and small business, through the issue of "fiduciary notes" which could be redeemed after the Depression. His Fiduciary Notes Bill was denounced as financially unsound by orthodox economists and the banks. It was eventually defeated in the Senate, which was still controlled by the conservative opposition. Theodore has been described as a visionary proto-Keynesian for this proposal, although it cannot be known what effect his measures would have had on the Depression had the bill been passed.
In March 1931, Lang's supporters in Parliament deserted the main body of Labor. Combined with the defection of Lyons and his supporters a few weeks earlier, this cost Scullin his majority. In November, the UAP and Langites rose to defeat the government on a non-confidence motion, and an election was held in December. Theodore had no base of support in Sydney and he lost his seat to Lang candidate Sol Rosevear. This ended Theodore's political career, although during the 1930s several offers were made to him to return.
Theodore went into business, becoming a business partner of Frank Packer in gold-mining ventures in Fiji and other enterprises, making him a rich man. He was chairman of directors of Packer's press company, Australian Consolidated Press, and director of several other companies.
During World War II Theodore served the Curtin and Chifley governments as Director of the Allied Works Council, which was established to undertake works requested by Allied Forces in Australia during World War II. After the war his health declined and he died of hypertensive cardiovascular disease on 9 February 1950 at Edgecliff, Sydney.
In 1909 Theodore married Esther Mahoney and they had four children. After Ted's death his son John assumed charge of the family's joint business interests with Frank Packer and he became the first managing director of Channel 9.
Subsequent Labor leaders such as Gough Whitlam regarded Theodore as a potentially great "lost leader" of the Labor Party, although it is unlikely that he could have become party leader after the Mungana Affair.