Roberts B. Owen, third from left, next to Secretary of State Warren M. Christopher. (Aric R. Schwan/State Department)

Roberts B. Owen, a Washington attorney who served as the State Department’s legal adviser during the Iran hostage crisis and played a crucial role as a negotiator and arbitrator at the end of the Bosnian war, determining the future of one of Bosnia’s most strategically important regions, died March 10 at his home in the District. He was 90.

The cause was congestive heart failure, said a daughter, Lucy Owen.

Mr. Owen developed his reputation as a shrewd legal mind at the D.C. office of Covington & Burling, where he argued antitrust cases before the Supreme Court and also involved himself in local issues.

In 1970, he was part of the legal team that successfully halted the construction of Three Sisters Bridge, a contentious span across the Potomac River that would have carved up sections of Northwest Washington with a new freeway. The project was supported by federal officials but opposed by civic groups and the D.C. government, which favored public transit over new highway projects.

Two decades later, he successfully sued to stop a new highway project centered on Southeast Washington’s Barney Circle.

Mr. Owen is sworn in as a State Department legal adviser in 1979. (Robert Kaiser/State Department)

Mr. Owen, by then a partner at Covington, was named the State Department’s legal adviser — its chief legal officer and the head of a 120-person legal team — in late 1979, just a few weeks before a group of Iranian students seized the U.S. Embassy in Tehran, taking more than 60 Americans hostage.

Working alongside Deputy Secretary of State Warren M. Christopher, Mr. Owen pressed for their release through the International Court of Justice in The Hague and coordinated efforts to establish economic sanctions, freeze Iranian assets abroad and enlist the support of foreign nations.

“The aim was to convince Iran’s leaders that their holding of the hostages would create, for Iran, costs that far exceeded any possible value to be gained from their unlawful conduct,” he remembered in the 2010 book “Shaping Foreign Policy in Times of Crisis,” an analysis of State Department legal advisers by law professors Michael Scharf and Paul Williams.

The diplomatic and economic maneuvering succeeded, and after a set of agreements penned in part by Mr. Owen during his final days as legal adviser, the hostages were released in January 1981.

Mr. Owen returned to Covington but was asked back to the State Department in 1995 to serve as a senior adviser in Bosnia, where a civil war was raging among the nation’s Croat, Serb and Muslim populations.

The diplomat Christopher R. Hill, who served as a chief aide to negotiator Richard Holbrooke during the conflict, recounted in his memoir “Outpost” that Mr. Owen was rumored to have been added to Holbrooke’s negotiating team to serve as the “eyes and ears” for Mr. Owen’s friend Christopher, by then secretary of state.

“It was probably true,” Hill wrote, “but Bob” — as Mr. Owen preferred to be called — “also brought to the table a very sensible, straightforward drafting style that would eventually form the basis for the entire Dayton accords.”

The accords brought the war to an end in late 1995, but Mr. Owen remained involved in Bosnia for another four years as presiding arbitrator of a special International Court of Justice tribunal.

He was tasked with deciding the fate of Brcko, a town of about 45,000 that had been inhabited mainly by Croats and Bosnian Muslims, who adopted the name Bosniaks in the mid-1990s, until it was seized by Serb forces during the war.

The Dayton accords had split Bosnia into two entities — the predominantly Serb Bosnian Serb Republic and predominantly Bosniak-Croat Federation of Bosnia and Herzegovina — and both sought control of Brcko, which is located on a strip of land that connects the western and eastern sections of the Bosnian Serb Republic.

Unconventionally, Mr. Owen decided to slice the Gordian knot in two, ruling in 1999 that Brcko would be part of both entities but ruled by neither. Instead, the town would be overseen by a new local government with international supervision, existing as “a kind of condominium” within Bosnia, as one U.S. official told the New York Times.

The decision infuriated Bosnian Serbs, who feared that the loss of the region would split the Bosnian Serb Republic in two. Access between the entity’s western and eastern sections was never cut off, however, and Mr. Owen’s solution remains.

Roberts Bishop Owen Jr. was born in Boston on Feb. 11, 1926. His father was a lawyer.

Shortly after graduating from the private Phillips Exeter Academy, he persuaded his mother to sign a release allowing him to enlist in the Navy in 1943, despite his youth. He served for three years at the close of World War II, ferrying German prisoners of war to and from the United States, and he maintained a love of sailing all his life, completing no fewer than eight Bermuda Races on his yawl, the Madrigal.

Mr. Owen graduated from Harvard University in 1948 and earned a law degree from Harvard Law School in 1951 before attending Queens College Cambridge on a Fulbright scholarship.

He began his law career at Covington & Burling in 1952 and was named a partner in 1960. He retired from the firm in 1996 with the title of senior counsel.

In addition to his work for the State Department, Mr. Owen served in the early 2000s as vice chairman of the Zurich-based Claims Resolution Tribunal for dormant accounts, which arbitrated claims made by the families of Holocaust victims to assets deposited in Swiss banks half a century before.

His legal work also included membership in the Permanent Court of Arbitration at The Hague, which resolves international disputes, in the 1980s and 1990s.

Survivors include his wife of 50 years, Kathleen von Schrader Owen of Washington; three children, David Owen of Somerset, Md., Lucy Owen of Washington and William Owen of Conway, N.H.; and eight grandchildren.

In an interview for the book “Shaping Foreign Policy,” Mr. Owen was asked whether he had imagined that more than 25 years after the Iran hostage crisis there would still be no diplomatic relations between Iran and the United States.

With customary good humor he replied, “Things do have a way of dragging on in international law and diplomacy.”