Thomas Mensah in 2001. He became involved in public service when he joined the International Maritime Organization in 1968. Photograph: ITLOS

In early 2013, when the Philippines brought an international case against China for violations of its rights under the law of the sea, a five-member arbitral tribunal was selected, with Thomas Mensah appointed as president. With China making extensive claims to resources falling within a “nine-dash line” (the undefined, vaguely located demarcation line used by China but disputed by other countries) across vast swathes of the South China Seas, the case was politically explosive for the whole region and beyond.

Mensah’s appointment reflected the esteem with which he was held, a deep knowledge of the law coupled with a recognition of his pragmatism and principle, and his fearless independent spirit. Three years later, under his guidance, the tribunal rendered a unanimous and far-reaching award, decisively rejecting China’s claim. The decision offered a rare example of judgment free from fear of “great power” retribution.

Mensah, who has died at the age of 87, was the world’s leading judge and arbitrator on the settlement of international maritime disputes. In 1996, when the new international tribunal for the law of the sea (ITLOS) began to function in Hamburg, he was elected as one of its 21 judges, and then chosen by colleagues to serve as the tribunal’s first president. In that role he oversaw the adoption of the rules and procedures that formed the character of the tribunal, and have stood it in good stead ever since.

He spent 19 years at ITLOS, also serving as an arbitrator and, later, as an ad hoc judge at ITLOS and at the international court of justice. The cases he decided were varied. In 2001, when Ireland and Britain disagreed over a proposed new nuclear facility at Sellafield, in Cumbria, Mensah served as president of the tribunal. In a dispute on the delimitation of the Bay of Bengal, as judge ad hoc appointed by Bangladesh, he collaborated with his counterpart, appointed by Myanmar, and produced a joint opinion – a rare act in inter-state litigation by two “opposing” judges. In a dispute between Argentina and Ghana, he confirmed his spirit of independence by joining in a unanimous decision against his own country.

His final case, between Ghana and Ivory Coast, brought particular satisfaction, ending a long-running maritime boundary dispute. These and other decisions reflected his conviction that international courts and tribunals contributed to the public good by ensuring that “international disputes are resolved peacefully, for the benefit of the parties and the international community as a whole”.

Mensah was born in Kumasi, the capital of the Asante kingdom, at a time when it had already become incorporated into Britain’s Gold Coast colony. He attended Achimota school in Accra, at a time of growing nationalist consciousness, and then went to the University of Ghana as the country approached its independence, a formative period in his early life.

In 1959 he obtained a first-class degree in law at the University of London, then headed to Yale Law School in the US, where in 1964 he obtained a doctorate. His thesis, on the right of self-determination for colonies, focused on the role of international institutions, including courts, to achieve this objective. The subject was one of enduring personal interest, one to which he remained committed throughout his life.

From Yale he returned to Accra, to lecture in law at the University of Ghana, serving briefly as dean of the faculty of law. He might have enjoyed a successful academic career but opted instead for public service of a more practical form, in the international domain. In 1968 he returned to London to join the secretariat of the International Maritime Organization (IMO), the UN agency then based in Piccadilly, later on the Albert Embankment. He rose to the position of director of legal affairs and external relations, and assistant secretary general, in 1981. His time there coincided with growing environmental concerns, and he played a formative role in giving birth to a new subject area, the protection of the marine environment, with standards for ships and rules on liability for damage by oil pollution and other sources.

Retirement from the IMO in 1990 opened the door to a new life, based at the family home in London. He lectured and wrote, and in 1995 was appointed Ghana’s first high commissioner to South Africa. Presenting credentials to President Nelson Mandela was a significant moment. The following year he became a judge at the international tribunal for the law of the sea, subsequently being elected as its first president. This was the start of global renown as an international adjudicator.

Anyone who came across Tom sensed his special qualities, the dignity, intelligence and biting humour coupled with a gentle humanity, an interest in the person. With a superbly mischievous twinkle in his eyes, he had no interest in baubles and honours. Much more fun to sneak off to an Arsenal game, or eat with family and friends, or just talk about the state of the world.

Those who appeared before him as counsel appreciated the practical efficacy and intellectual acuity. Those who sat with him recognised his unique ability to forge agreement among disparate colleagues. One of his closest friends, his judicial colleague Judge Rudiger Wolfrum, explained what made it special to judge with him, particularly on the most contentious issues, such as the South China Seas: “Tom could listen, understand, show sympathy,” he told me. “He had his own views, but he never insisted upon them to the exclusion of the views of others.”

He is survived by his wife, Akosua, children, grandchildren and a great-grandson.

• Thomas Aboagye Mensah, international lawyer, born 12 May 1932; died 7 April 2020