Prior to his immigration to the United States, Meron was a legal adviser of the Israeli Ministry of Foreign Affairs.

In 1990, Meron served as a “Public Member” of the United States Delegation to the CSCE Conference on Human Dimensions in Copenhagen.

[4] For service to criminal justice and international Humanitarian Law, Queen Elizabeth II made him an Honorary Companion of "the Most Distinguished Order of St. Michael and St. George" (CMG) in 2019.

[6] That same year, he was also one of 17 honorees selected by One Young World and Vanity Fair for the inaugural Global Achievements List, cited for his contributions "for peace, justice and strong institutions" (UK March 2019 issue).

[7] In 2024, Meron was part of an expert panel that recommended ICC chief prosecutor Karim Ahmad Khan issue arrest warrants against Israeli Prime Minister Benjamin Netanyahu, Defense Minister Yoav Gallant and three Hamas leaders (Yahya Sinwar, Mohammed Deif, Ismail Haniyeh) on suspicion of war crimes and crimes against humanity in the Israel–Hamas war.

[8][9][10] After Israel's victory in the Six-Day War of June 1967, Meron, as legal counsel to the Israeli Foreign Ministry, wrote a secret memo[11][12][13] for Prime Minister Levi Eshkol, who was considering re-establishing the Jewish settlement of Kfar Etzion, which had been destroyed by Arab forces in 1948.

Meron's memo concluded that creating this new settlement in the Occupied Territories would be a violation of the Fourth Geneva Convention.

[14] Judge Theodor Meron presided over the Appeals Chamber in the case of Radislav Krstic which confirmed that systematic murder of over 8,000 men and boys in Srebrenica constituted genocide.

He visited the Srebrenica Genocide Memorial in 2004 and stated: "...Where these requirements are satisfied, however, the law must not shy away from referring to the crime committed by its proper name.

The letter claimed Meron had raised the degree of responsibility that senior military leaders should bear for war crimes committed by their subordinates, to the point where a conviction has become nearly impossible.

[16] In August 2013, a chamber appointed by the ICTY Vice-President found by majority that Judge Harhoff had demonstrated an unacceptable appearance of bias in favour of conviction.

In the Judgment of the International Court of Justice of 3 February 2015, the Court, which is the principal judicial organ of the United Nations, expressed agreement with the ICTY majority judgement in the case of Ante Gotovina and Mladen Markač, which was at the center of Harhoff's criticism of Meron, who presided over the Gotovina and Markač appeal.