Dr. Crossman served as President of the White Mountains Medical Society; and in 1903 was elected to the New Hampshire General Court, the state legislature.

Crossman was assigned to the 76th Field Artillery at Fort Ethan Allen, Vermont and promoted to First Lieutenant on October 26.

From January 9 – March 1919 Crossman attended the Battery Officer's course in the Field Artillery School of Fire at Fort Sill, Oklahoma.

He returned to the 4th Field Artillery at Camp Stanley, Texas as a Battery Commander, and transferred to the Reserve Corps (inactive) on June 11, 1919.

[8] In 1928, at the invitation of Henry L. Stimson, Governor-General of the Philippines, Crossman took a leave of absence from the firm and became his legal aide.

[9] At the conclusion of his assignment, the Crossman's traveled in China, traversed the Soviet Union via the Trans-Siberian Railway, and spent some time in Moscow, Warsaw, Prague and other cities in Europe.

He was and attached to General Headquarters Southwest Pacific Area (GHQ SWPA) and United States Army Forces in the Far East (USAFFE).

As Executive Officer and deputy to Brigadier General Courtney Whitney, Chief of the Civil Affairs Section.

Crossman's role was complicated as MacArthur's professional staff was jealous of him as he had known the General socially in the Philippines when he worked for Stimson.

On July 27, 1945, Crossman was promoted to Colonel and was requested by Stimson, then the United States Secretary of War, to be MacArthur's representative on his personal staff in Washington, D.C.

In this capacity, he performed a liaison function and was ordered to help rewrite the Japanese Occupation Directive which incorporated his experience during the Philippine campaign.

In 1947, Crossman was appointed American Chairman of the Joint American-Philippine Finance Commission, acting as personal representative of President Harry S. Truman, with the rank of Minister.

The Commission studied Philippine financial, monetary, and fiscal problems and recommended a shift from the dollar exchange standard to a managed currency system.

The new Central Bank of the Philippines Bangko Sentral ng Pilipinas implemented the Commission's recommendations.

[12] In 1947, Crossman returned to practicing law at Davis, Polk in New York; and as outside counsel continued to advise major institutional clients such as Morgan Stanley, the Northern Pacific Railroad and Freeport Sulfur.