O'Brien is notable as Boston's first Irish and Catholic mayor, having emigrated from Ireland to America in the early 1830s.

Born in Ireland on July 13, 1827, O'Brien emigrated to America in the early 1830s as a young child.

He dropped out of public school at the age of twelve to pursue a career in newspapers,[3] eventually becoming the editor of the Shipping and Commercial List.

He was a champion of the working class and helped pass legislation regulating the pay of men under city contractors.

[4] O'Brien was appointed by Mayor Nathan Matthews Jr. to Boston's Board of Survey, which was tasked with planning the city's streets.