When Hugh O'Brien was sworn in as Boston's first Irish-born mayor in 1885, it marked the beginning of a new era in Boston politics. The city had long been controlled by native-born Protestants—generally called "Yankees"—most of whom had a stereotypical view of Irish immigrants as poor, ignorant, undisciplined, and under the thumb of the Catholic Church. But the Irish-born population of Boston was exploding, growing from 2,000 in 1820 to 7,000 by 1830. By 1855, it was 44,000; 25 years later, more than 70,000 Irish lived in Boston. By 1885, the Irish were over 40% of the city's population. They were the largest group of foreign-born residents and outnumbered the native-born Yankees.

Although the Yankees maintained their economic and social dominance of Boston, in the years after the Civil War they were forced to recognize the potential political power of the city's Irish Catholics. During the middle decades of the nineteenth century, the native-born vote grew only 14.7% while the largely Irish foreign vote grew 195%. The typical Yankees' attitude towards Irish immigrants turned from scorn to fear. To break into mayoral politics, the Irish needed a candidate who could quiet these fears. Hugh O'Brien was just such a man.

Born in Ireland in 1827, he moved with his family to Boston when he was five—well before the potato famine sent waves of impoverished Irish men and women to Boston. He spent seven years in the Boston public schools, and was apprenticed to a printer at the age of 12.

Working first for the newspaper Boston Courier and then for a Boston printer, Hugh O'Brien excelled at the printing business. He was made a foreman when he was only 15. He started his own publication, Shipping and Commercial List, and was soon successful enough to become a respected member of the Boston business elite.

O'Brien's business success, and the wealth and position it gave him, drew the attention of Patrick Maguire, publisher of The Republic newspaper and the unofficial head of Irish politics in Boston. He orchestrated the election of Hugh O'Brien to the city's Board of Alderman. O'Brien's seven years on the Board were characterized by fiscal restraint, hard work, and an utterly respectable demeanor. Articulate, responsible, and well-to-do, O'Brien defied Yankee stereotypes of Irishmen.

By 1883 Patrick Maguire had decided that the time had come for Boston to elect an Irish-born mayor. He devised a two-part strategy. O'Brien would be the public face of the campaign, an able public official who criticized the previous administration for increasing taxes. O'Brien's pledge to reduce the tax rate without cutting city services appealed to the Yankee tradition of frugality. Meanwhile, behind the scenes, Maguire developed a system of Irish ward bosses who visited each household in the neighborhood and made sure that every eligible Irishman voted for O'Brien. Hugh O'Brien swept 15 of Boston's 25 wards. On December 10, 1884, he became the first Irish Catholic to be elected Mayor of Boston.

Many in the former administration feared that O'Brien's election would be followed by a wave of Irish appointments to key positions. In the weeks before inauguration day, the state legislature passed bills to limit the new mayor's power. To prevent O'Brien from appointing friends and relatives to city jobs, the legislature passed a civil service law to regulate the hiring and promotion of state and city employees. Liquor licensing and oversight of the police department were removed from the mayor's jurisdiction and placed under the control of a special commission appointed by the governor.

O'Brien surprised the opposition by governing the city in a conservative and honest way. He did indeed cut tax rates; he also widened streets, established the commission that hired Frederick Law Olmsted to design the Emerald Necklace park system, and built the new Boston Public Library in Copley Square. He disarmed his critics by enlisting Yankee and Republican businessmen to serve on the committees overseeing these projects.

Meanwhile, during O'Brien's four terms in office, the system of Irish neighborhood bosses blossomed and the Irish political machine matured. While O'Brien appeared to the Yankees as the ideal, disinterested civil servant, the Irish community saw his role as meeting the day-to-day needs of beleaguered constituents in exchange for political patronage. Over the next half century, the Boston Irish would build a formidable political machine that consistently turned out voters in support of men who would provide jobs, assistance, and protection for their Irish constituents. In return, these politicians could expect to have the loyalty—and the votes—of their fellow immigrants.

Because Hugh O'Brien did not publicly endorse such "ethnic politics," he retained the support of both native- and Irish-born Bostonians. Within a generation, "Honey Fitz" Fitzgerald, James Michael Curley, and other Irishmen would follow O'Brien into City Hall. Their irreverent, brash, and confrontational brand of politics would pit the Irish neighborhoods against the downtown Yankees. By the 1930s, the Irish political machine dominated Boston politics to a degree that would have been inconceivable on the day that Hugh O'Brien was sworn into office.