After three months with Ware & Van Brunt he entered the Massachusetts Institute of Technology for a special course in architecture.

After another year with Van Brunt he moved to the studio of Henry Hobson Richardson in Brookline in March 1883.

[1] When Richardson died in April 1886 Coolidge and two other senior employees, George Foster Shepley and Charles Hercules Rutan, took charge of the studio and its uncompleted work.

In June 1886 the three formed a formal partnership, Shepley, Rutan & Coolidge, to succeed to Richardson's practice, and in 1887 moved the office to Boston.

[1][4] One of his grandsons, Daniel Jones Coolidge (1924–1992), was an architect who practiced with Shepley, Bulfinch, Richardson & Abbott from 1954 until his retirement in 1990.

In 1906, when the firm's Harvard Medical School was dedicated, Coolidge was awarded an Honorary Doctor of Arts.