Duggan was the state president of the Shop Assistants Union and also president of the Toowoomba branch of the ALP at just 21 years of age. Three years later, he won the seat of Toowoomba for the Labor Party in the 1935 by-election to replace the sitting member, Evan Llewelyn. He went on to represent Toowoomba for the next 22 years. In 1947 Duggan was promoted to the Transport portfolio, a post he retained under Ned Hanlon and Vince Gair. From 1953 onward, he was also deputy premier. He lost his cabinet posts when Gair led his most of his ministry out of the ALP and formed the Queensland Labor Party in 1957. Duggan, the sole minister not to defect, succeeded Gair as state Labor leader. When Gair's government sought supply, Duggan led Labor to vote against the motion. The Coalition, led by Frank Nicklin, also voted against the motion and brought the government down. At the ensuing election, Duggan led Labor to second place in the overall vote and 20 seats. However, since every ALP MP was opposed by a QLP challenger and vice versa, the Labor vote was hopelessly divided. The first past the post system denied the Labor forces the option of directing preferences to each other even if they'd wanted to. Taking advantage of the large number of three-cornered contests, Nicklin led the Coalition to a decisive victory, taking 42 seats against only 31 for the two Labor forces combined. Duggan himself was defeated by LiberalMervyn Anderson. The Labor Party was eager to get Duggan back into the legislature. Despite having no connection with the electorate of Gregory in central Queensland, he contested a by-election for that seat. To no avail: he was defeated by a local Country Party candidate, Sir Wallace Rae. In March 1958, Duggan's successor as state Labor leader, Les Wood, suddenly died. Duggan became the ALP candidate for Wood's seat of North Toowoomba. After scoring a comfortable win in the constituency, he returned to the Labor leadership. North Toowoomba was abolished before the next state election and Duggan followed most of his constituents into the new seat of Toowoomba West. He represented the electorate until 1969 when he retired from parliament. Duggan served as opposition leader until 1966, a time filled with difficulty. He not only had to deal with the presence of the QLP, which merged into the Democratic Labor Party in 1962, but was faced with particularly strenuous infighting within what was left of the ALP's parliamentary representation. His position was made even more difficult with the reintroduction of preferential voting in 1963. As a result, the DLP not only siphoned off votes from the ALP, but directed its preferences to the Coalition. Thus, the Coalition won elections in 1963 and 1966 without serious difficulty. Having left state politics, Duggan remained in public life. He was elected to the Toowoomba City Council and was Mayor of Toowoomba in 1981.