The Stena Impero was seized by Iranian forces Friday while transiting the vital Strait of Hormuz — providing the first test for Johnson on becoming prime minister Wednesday afternoon.
Though Johnson has said military action is not on the agenda, it's unclear what strategy he will pursue to get the ship out.
During the final days of Theresa May's premiership, Britain has sought to cobble together an as-yet-undefined "Europe-led force" that could protect shipping in the waterway, which transports one-fifth of the world's crude oil supply.
It's not clear whether the plan formulated by British Foreign Secretary Jeremy Hunt to form an alliance with France, Germany and other European nations to keep the waterway open is gaining traction.
French Foreign Ministry spokeswoman Agnes Von der Muhll has spoken of putting in place an "appropriate means of surveillance" aimed at "increased understanding of the situation at sea" to keep shipping moving in the face of the threat of Iranian intervention.
There was one hopeful sign Wednesday: A large British-flagged vessel transited the Strait of Hormuz and arrived at a port in Qatar in the first such passage made by a British ship of its size since the Stena Impero seizure last week, according to maritime tracking data.
In Iran, President Hassan Rouhani suggested a way to defuse the crisis would be for Britain and Gibraltar to release an Iranian supertanker seized off the coast of Gibraltar on July 4 on suspicion that it was violating European Union sanctions by delivering oil to Syria.
He said that if the British let the supertanker go, then "they will receive a proper answer by Iran." Britain has rejected any comparison and defended its seizure of the supertanker as completely legal.
Johnson dealt with Iran's top diplomats during his time as foreign secretary, forming a working relationship with his Iranian counterpart, Mohammad Javad Zarif.
But his tenure was marred by an unfortunate comment he made in 2017 about Nazanin Zaghari-Ratcliffe, a British-Iranian woman held by Iran on accusations that she was plotting against the government.
Johnson mistakenly said in 2017 that she was in Iran to teach journalism — which appeared to undercut her position and make it easier for Iran to impose a prison sentence. Johnson later apologized and said in Parliament he had "no doubt" she was on holiday in Iran. Zaghari-Ratcliffe is still imprisoned under harsh conditions.
Tensions with Iran have been ratcheted up in recent months, in large part because of President Donald Trump's decision to withdraw support for a carefully negotiated deal designed to prevent Iran from obtaining nuclear weapons, but Johnson — in the weeks before taking office — has refrained from saber rattling. He said ten days ago he does not favor military action against Iran.