Following the end of his term as president, Adamkus remained involved in international development, and is a member of the European Academy of Diplomacy.

Adamkus also raised concerns about other Soviet activities in occupied Lithuania to United Nations Secretary General Dag Hammarskjöld in 1958, and to President John F. Kennedy in 1962.

In 1981, he was appointed regional administrator by President Ronald Reagan, and was responsible for all air, water, hazardous waste, and other pollution control programs in Illinois, Indiana, Michigan, Minnesota, Ohio, and Wisconsin.

In 1985, President Reagan presented him with the Distinguished Executive Presidential Rank Award – the highest honor that can be bestowed upon a civil servant.

Upon his retirement, he received a congratulatory letter from President Clinton and a Distinguished Career Award from EPA Administrator Carol Browner.

EPA Region 5 presented him with the newly established "Valdas V. Adamkus Sustained Commitment to the Environment Honor Award".

Doubts arose whether Adamkus was eligible to run for the presidency due to having spent over half a century abroad, raising the possibility that he might not meet minimum residency requirements.

However, the court resolved the case in Adamkus' favor, and no other obstacles remained other than his U.S. citizenship, which he officially renounced at the American Embassy in Vilnius.

The Director-General of UNESCO, Koïchiro Matsuura, noted that Adamkus was named as Ambassador "in recognition of his dedication to the Organization's aims and ideals and with a view to benefiting for the construction of knowledge societies from his wisdom and extensive experience in many of UNESCO's areas of concern, in particular promotion of social development, cultural diversity, dialog and international cooperation.

President Adamkus recalled in an interview that "when I asked what we could do to help, Kuchma said the friends of the Ukrainian people should drop whatever they were doing and come to Kiev immediately.".

Valdas Adamkus and his Estonian counterpart Arnold Rüütel rejected an invitation to participate in a commemorative celebration of the end of World War II in Europe in 2005.

President Adamkus expressed the view that the war's end, in Lithuania, marked the beginning of a fifty-year Soviet occupation and repression.

In response, on 22 July, the United States Congress unanimously passed a resolution that Russia should "issue a clear and unambiguous statement of admission and condemnation of the illegal occupation and annexation by the Soviet Union from 1940 to 1991 of the Baltic countries of Estonia, Latvia, and Lithuania",[14] but Russia refused.