Jenkins was born on July 18, 1834, in Saratoga Springs, New York, United States, the son of Edgar Jenkins, a merchant, and Mary Elizabeth (Walworth) Jenkins. His mother's father was Reuben Hyde Walworth, a justice of the supreme court of New York and a distinguished chancellor of that state.
James was educated in the private schools of his native city and commenced the study of law in the offices of Ellis, Burrill & Davison, New York City.
Upon his admission to the bar Jenkins immediately commenced the practice of his profession and two years later (1857) removed to Milwaukee. During the time Jenkins was engaged in practice he was successively the law partner of Jason Downer, later a justice of the Supreme Court, Edward G. Ryan, later chief justice of the supreme court of Wisconsin, Senator Matthew Hale Carpenter, Theodore B. Elliott, and General Frederick C. Winkler. It would be difficult to select from the roll of Wisconsin lawyers a more distinguished group. At the time of his appointment to be United States district judge for the eastern district of Wisconsin, he was generally recognized as the leader of the bar.
When Walter Q. Gresham resigned the office of United States circuit judge for the seventh judicial circuit to become a member of President Cleveland's cabinet Jenkins was appointed to succeed him and continued to serve in that capacity until his retirement to private life in 1905. Jenkins' name is associated with two important cases: Pillsbury vs. Pillsbury Washburn Company, Ltd. , and Farmers Loan & Trust Company vs. Northern Pacific Railway Company.
The first case, which involved matters generally classified under unfair competition or unfair trade, remains a landmark in the history of American jurisprudence. The principles there laid down, amplified and extended, have so far governed the law of that subject.
The second case, generally known as the Northern Pacific receivership, involved Jenkins in a controversy with Congress. John C. Spooner, counsel for the receiver, filed a petition in the court over which Jenkins presided. Upon the petition the court issued the famous injunctional or strike order which enjoined the employees then in the service of the receiver from combining and conspiring to quit the services of the receiver with the object and intent of crippling the property in their custody. This order became the subject of investigation by a subcommittee of the House of Representatives. The committee attempted to summon Jenkins before it. He declined to comply on the ground that the committee was not authorized to inquire into any matters affecting his personal or official integrity. The case was later appealed to the Supreme Court of the United States and there, with very slight modification, the decree was affirmed.
After his retirement from the bench, Jenkins was for seven years (1908-1915) dean of the law school of Marquette University, Milwaukee. Throughout his life he was an influential leader of the Democratic party and was its candidate for governor in 1879. In 1881 he received the vote of his party in the legislature of Wisconsin for United States senator. While his party in the state was in a hopeless minority, his interest in it never waned. He was active in its councils and gave much of his time and energy to its support upon the stump. Jenkins died in Milwaukee, Wisconsin, on August 6, 1921.
Jenkins was of medium height, commanding presence, and distinguished bearing, and he enjoyed to an unusual degree a deserved popularity with the people of his state.
Jenkins was married in 1870 to Alice Mary Miller, daughter of Andrew Gilbraith Miller, then United States district judge for the eastern district of Wisconsin.
