Her great-grandfather, Michael T. O'Brien, was the proprietor of the family funeral home in Easthampton, Massachusetts, and first elected to the state legislature in 1930.

He then ran for Congress in 1976, losing to the incumbent Silvio Conte; he returned to the Governor's Council after the 1978 election and remained in office until his death in 2004.

[citation needed] O'Brien worked for a large Boston law firm before her father alerted her to an open seat in the state legislature, which she won.

[2] She won the Democratic gubernatorial primary in 2002, defeating State Senate President Thomas Birmingham, former Democratic National Committee and American Israel Public Affairs Committee chair Steven Grossman, former United States Secretary of Labor Robert Reich, and former nominee for lieutenant governor Warren Tolman.

[7] In August 2022, State Treasurer and Receiver General Deb Goldberg appointed O'Brien as Chair of the Cannabis Control Commission.

[citation needed] Shannon O'Brien Suspended from Cannibis Control Commission, story by Matt Stout, Boston Globe https://www.bostonglobe.com/2023/09/15/metro/shannon-obrien-suspended-cannabis-control-commission-deborah-goldberg/?s_campaign=breakingnews:newsletter