New York State Senator Robert Lansing (1799–1878) was his grandfather; Chancellor John Lansing, Jr. and State Treasurer Abraham G. Lansing were his great-granduncles.

Robert Lansing was born in Watertown, New York in October 1864, the son of John Lansing (1832–1907) and Maria Lay (Dodge) Lansing. He graduated from Amherst College in 1886, studied law, and was admitted to the bar in 1889.

In 1890, Lansing married Eleanor Foster, the daughter of Secretary of State John W. Foster. Eleanor's older sister Edith was the mother of John Foster Dulles, who also became Secretary of State, Allen Welsh Dulles who served as Director of Central Intelligence, and Eleanor Lansing Dulles, an Economist and high level policy analyst and advisor for the State Department.

Lansing was associate Editor of the American Journal of International Law, and with Gary M. Jones was the author of Government: Its Origin, Growth, and Form in the United States (1902). He also wrote: The Big Four and Others at the Peace Conference, Boston (1921) and The Peace Negotiations: A Personal Narrative, Boston/New York (1921).

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 1892-1893, as counsel for the United States Bering Sea Claims Commission in 1896-1897, as the government's Lawyer before the Alaskan Boundary Tribunal in 1903, as counsel for the North Atlantic Fisheries in the Arbitration at The Hague in 1909-1910, and as agent of the United States in the American and British Arbitration in 1912-1914. In 1914 Lansing was appointed counselor to the State Department by President Woodrow Wilson.

Lansing advocated "benevolent neutrality" at the start of World War I. 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. william Jennings Bryan resigned as Secretary of State following Wilson's second note, which Bryan considered too belligerent. Lansing replaced Bryan, and said in his memoirs that following the Lusitania tragedy he always had the "conviction that we would ultimately become the ally of Britain".

In 1916 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 (DSS).

In 1919, Lansing 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, for whom participation in the League of Nations was a primary goal. During Wilson's stroke and illness, Lansing called the cabinet together for consultations on several occasions. In addition, he was the first cabinet member to suggest that Vice President Thomas R. Marshall assume the powers of the presidency. Edith Wilson was displeased by Lansing's independence, and requested Lansing's resignation in 1920.

After leaving office Lansing resumed practicing law. He died in New York City on October 30, 1928, and was buried at Brookside Cemetery in Watertown, New York.