In that country he played an important role in negotiating settlements in boundary disputes with British Burma and the French colony of Vietnam.

He was effective in helping to develop a native government, a state administrative system and a criminal code.

[3][5] As General Advisor, Westengard was enormously influential in effecting a wide variety of reforms and civil improvements on behalf of the government; among these, in addition to those already mentioned, were negotiating foreign loans, drafting legislation and modifying existing laws, planning water works, restructuring the kingdom's finances and system of revenue, negotiated foreign treaties, and proposed ambassadors and ministers.

For the coronation of King Rama VI in 1910, Westengard brought together the largest gathering of European royalty in Asia then known.

[2] After twelve years in Siam, Westengard resigned from his service to the government in June 1915 and returned to Harvard to take up teaching law again.

[2] Considering the differences between teaching law and his work in diplomacy, Westengard could rightly boast that the latter afforded him considerably more honors, as he once wrote in a letter: I remember well that I dined one night with [Harvard University] President Eliot, when, pointing out the happiness of the law teacher's lot, he said that the practicing lawyer was a man "whose name was writ in water."

Shortly after he was informed that the Siamese government wished him to be their representative at the Paris Peace Conference after World War I, Westengard died in Cambridge, Mass., on September 17, 1918, after a brief illness.