Born in Watertown, New York, he graduated from Amherst College in 1886 and was admitted to the bar in 1889. From then until 1907 he was a member of the law firm of Lansing & Lansing at Watertown. An authority on international law, he served as associate counsel for the United States, in the Bering Sea Arbitration in 1892-93, as counsel for the United States Bering Sea Claims Commission in 1896-97, as solicitor for the government before the Alaskan Boundary Tribunal in 1903, as counsel for the North Atlantic Fisheries in the Arbitration at The Hague in 1909-10, and as agent of the United States in the American and British Arbitration in 1912-14. In 1914 Lansing was appointed by President Wilson counselor to the State Department.

World War I

Lansing advocated "benevolent neutrality" in World War I, and eventually American participation. Following the sinking of the RMS Lusitania on 7 May 1915 by the German submarine U-20, Lansing backed Woodrow Wilson in issuing three notes of protest to the German government issued on 13 May, 9 June, and 21 July. William Jennings Bryan resigned as Secretary of State following Wilson's second note, in which Wilson rejected the German arguments that the British blockade was illegal; was a cruel and deadly attack on innocent civilians; and that the Lusitania had been carrying munitions. Bryan considered the note too provocative and resigned in protest after failing to moderate it, to be replaced as Secretary of State by Lansing, who later said in his memoirs that following the tragedy he always had the "conviction that we would ultimately become the ally of Britain".

In 1916, using funds discretionary to himself, Lansing hired a handful of men who became the State Department's first special agents in the new Bureau of Secret Intelligence. These agents were initially used to observe the activities of the Central Powers in America, and later to watch over interned German diplomats. The small group of agents hired by Lansing would eventually become the U.S. Diplomatic Security Service many years later.

In 1919, he became the nominal head of the US Commission to the Paris Peace Conference. Because he did not regard the League of Nations as essential to the peace treaty, Lansing began to fall out of favor with Wilson. During Wilson's stroke and illness, Lansing called the cabinet together for consultations on several occasions. In addition, Lansing was the first cabinet member to suggest that Vice President Thomas R. Marshall assume the powers of the presidency. Wilson was disturbed by Lansing's independence, and Lansing resigned in 1920 at Wilson's request. Afterward, he practiced law in New York City.

His nephews include John Foster Dulles, who also became a U.S. Secretary of State, and Allen Welsh Dulles, a Director of Central Intelligence.

He became associate editor of the American Journal of International Law, and with Gary M. Jones was author of Government: Its Origin, Growth, and Form in the United States (1902). He wrote: The Big Four and Others at the Peace Conference (1921) and The Peace Negotiations (1922).