Mason was born August 23, 1839 in
Bridgeport,
Connecticut. He was the son of
Roswell B. Mason and Harriet L. Mason. Mason's father moved the family to
Chicago,
Illinois in the early history of Chicago (in the years it was still a village, rather than a city). A civil engineer, his father was involved in several emerging business enterprises in Chicago and eventually served the city's mayor. Mason himself completed college preparatory studies in Chicago, attending
Yale University and graduating in 1860. Two of Mason's siblings,
Henry B. Mason and
Alfred B. Mason, also later attended Yale (in the graduating classes of 1870 and 1871, respectively). ==Legal career==