McMillan was born in
Brownsville, Pennsylvania in 1826. His parents Thomas Long and Jane McMillan (née Gormly) were of Scottish and Irish ancestry. He attended the
University of Pittsburgh (then known as the
Western University of Pennsylvania) in
Pittsburgh, Pennsylvania and studied law under
Edwin M. Stanton, graduating in 1846. In 1850 he married Harriet Butler. McMillan and his wife moved to Minnesota in 1852, initially settling in
Stillwater and later moving to
St. Paul. When Minnesota achieved statehood in 1858 McMillan was named as the first judge of the newly formed Minnesota District Court. During the
Dakota War of 1862 McMillan served as a Second Lieutenant in a militia unit known as the Stillwater Frontier Guards which was only briefly active. In 1864 he was appointed to the Minnesota Supreme Court as one of two new justices to replace the recently resigned Justices
Charles Eugene Flandrau and
Isaac Atwater. In 1874 he was promoted to
Chief Justice to replace
Christopher G. Ripley who had also resigned. In 1875, the
Minnesota Legislature elected him to serve as U.S. Senator. He served in the
44th,
45th,
46th,
47th,
48th, and
49th United States Congresses from March 4, 1875, to March 3, 1887. McMillan did not stand for re-election in 1886 and returned to his law practice after the end of his term. He died in
Saint Paul on October 3, 1897. He is buried in Oakland Cemetery in St. Paul, Minnesota. ==Samuel James Renwick McMillan family papers, 1818-1956. Minnesota Historical Society==