He studied at the University of the Pacific (then located in Santa Clara) from 1858 to 1862.
Maclay left Ohio in September 1855 and spent two years hunting and trapping on the great plains and in the Rocky Mountains, arriving at last in California in September 1857. He was admitted to the bar in 1865, and moved to Los Angeles in 1867. In 1871, he was named a judge of the Court of California for Los Angeles and San Bernardino Counties.
In 1874 he began the first successful rail transit company in Los Angeles, building a horsecar line from The Plaza to 6th and Pearl (now Figueroa Street) Streets.
He was a Republican. Los Angeles was a frontier town in the early 1870s, when a group of public-spirited citizens led by Judge Robert Maclay Widney first dreamed of establishing a university in the region. The gift provided land for a campus as well as a source of endowment, the seeds of financial support for the nascent institution.
Robert M. Widney is interred in Angelus-Rosedale Cemetery, Los Los Angeles The bronze monument, sculpted by Christopher Slatoff, stands on campus at the entrance of the Widney Alumni House.