Beginning in 1901, he was a senior partner of Coolidge & Carlson, an architectural firm noted for large institutional projects.

In 1891, he traveled back to Europe where he was admitted to the Beaux-Arts de Paris and joined the atelier of Henry Duray, a patron popular with American students.

[1] Their largest work together was Randolph Hall, a private Harvard dormitory financed by the Coolidge brothers.

[3] Coolidge was generally hands-off in regard to the work of the firm, more frequently providing criticism to the architects and drafters in his employ.

In 1917, Woodrow Wilson appointed him regional member of the National Library War Council which had the responsibility of distributing reading materials to soldiers at home and at the front.

[5] In 1899, Coolidge was appointed to the board of trustees of the Museum of Fine Arts as the representative of the Boston Athenæum.

The couple had eight children:[7] After his return to Boston, Coolidge and his wife lived on Marlborough Street near his parents' home, moving to Brookline in 1904.

[3] In 1925–26, he served a term in the New Hampshire House of Representatives,[10] and in 1926, he and his wife were among the founders of the Sandwich Home Industries, a local craftsmen's cooperative formed primarily through the efforts of Mrs.