Admitted to the Minnesota bar in June 1895, James E. O'Brien practiced law in Minneapolis for the next half century. He died on May 19, 1947 at age seventy-seven. In a memorial to him the following year for the Hennepin County Bar Association, Kimball B. Devoy recalled his friend with admiration:
"His professional life was characterized by a success unparalleled in the Northwest. Success in the financial way was his, but far greater and far more deeply satisfying, his greatest achievements lay in the respect of Judges, in the esteem of his fellow lawyers, in the love and implicit confidence of his clients, and, within his own heart, in the certain knowledge that that which he had undertaken was done to the fullest of his ability, physical and mental."