William “Mo” Cowan, who served a brief term as an interim US senator earlier this year, has accepted a position with a powerhouse Boston law firm as the chief operating officer of its government relations wing.

After completing a fellowship at Harvard’s Institute of Politics in November, Cowan will become chief operating officer at ML Strategies and rejoin the Mintz Levin law firm, where he worked before signing on as Governor Deval Patrick’s chief legal counsel in 2009.

Cowan told the Globe he plans to split time between Boston and Washington, D.C., helping clients “navigate some of the tricky challenging waters, particularly those that interact with government.”

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He said he had entertained a number of private-sector offers, but, “This seemed like the most exciting and enticing opportunity.”