In January 1865, while still a boy, James joined the Confederate army. The close of the war found his family in reduced circumstances, but an uncle, James Grant, a lawyer of Davenport, Iowa, undertook to educate his nephews, and in December 1870, James went to Davenport.
He soon entered the Iowa State College of Agriculture, later spent a year at Cornell University studying civil engineering, and in 1874, went to the School of Mines at Freiberg, Saxony, where he studied mining and metallurgy for two years.
Equipped for his life-work, Grant traveled round the world seeking a place in which to practise his profession and selected Colorado.
After an unfortunate start in 1876, in Clear Creek and Gilpin counties, he went to Leadville and in partnership with his uncle built the Grant Smelter. It opened for business on October 1, 1878, with James Benton Grant as manager.
With the growth of mining in Leadville the smelter prospered and its manager became a well-known citizen.
The smelter in Leadville was burned in 1882 and Grant moved to Denver, where, after a merger with the Omaha Smelting Company of Nebraska, the Omaha & Grant Smelter was opened in 1883.
In 1899, the new company was consolidated with others to form the American Smelting and Refining Company, the largest of its kind in the United States. Grant was a director of the new concern.
Although his capital was largely invested in smelting, he had a substantial interest in Leadville mines and was one of the engineers who launched the Yak Tunnel in the same district.
He was also one of the organizers, in 1884, of the Denver National Bank and became its first vice-president, which position he held until his death.
In 1882, Grant was induced, after much persuasion, to run for governor on the Democratic ticket. In his favor he had means, personal popularity, and a reputation for absolute integrity.
On the other hand, he was young, a Democrat in a Republican state, and an ex-Confederate soldier in a Union district. The Republican and Greenback-Labor parties made unfortunate nominations, however, and Grant was elected, the first Democratic as well as the youngest governor of Colorado.
Grant's administration has been characterized as “a quiet, strong, tranquil government, almost without striking incident”. This was the beginning and end of his career in high political office; he had no ambitions in that direction.
Throughout his residence in Denver, however, he was interested in educational matters. In 1891, he was elected a member of the Denver Board of Education, was chosen its president the following year, and ably filled that position from 1892 to 1899.
He served also as one of the trustees of the University of Denver from 1884 to 1904, and was one of the organizers of the Colorado Scientific Society.
In 1902, heart trouble caused him to curtail his activities. His death occurred at Excelsior Springs, Missouri, in his sixty-fourth year.
During his term, Grant developed and expanded Colorado's mining industry, advocated legislation that authorized the construction of the Colorado state capitol building in Denver and improved Colorado's trade industries.
On January 19, 1881, Grant was married to Mary Matteson Goodell of Springfield, Illinois.