The lowering of the containment device played out 80 kilometres from the coast of Louisiana.
BP, which owns the leaking well, has said it does not expect the container to be fully operational until at least Sunday or Monday.
About 400 metres away is the wreckage of the drilling rig Deepwater Horizon which BP was leasing when it exploded on April 20.
The operation to contain the spill came as oil sheen was seen spilling on islands off the coast of the southern US state of Louisiana, the US coast guard said.
"Teams have confirmed oil on Freemason Island," Petty Officer Connie Terrell of the US coast guard, said on Thursday.
"It is at the south end of the Chandeleur Islands. It is largely sheen with no evidence of medium or heavy oil."
Oil sheen is the shimmering thin layer of crude settled on top of water.
John Curry, BP's spokesman, said that three emergency response teams were sent to the island, some 50km offshore, and were deploying inflatable booms to try to protect the prime marsh and wildlife area.
"This is the first confirmation of shoreline impact we have had," he said.
"We are doing everything we can to make sure a major impact doesn't happen."
The Chandeleur Islands form the easternmost point of Louisiana and are part of the Breton National Wildlife Refuge - the second oldest refuge in the US and home to countless endangered shorebirds.
Al Jazeera's Cath Turner, reporting from the Gulf of Mexico, said that water off the Mississippi coastline was now pinkish-reddish in colour, having been stained by oil and dispersant.
"The mostly fine weather has helped authorities contain the spill, but there's no telling how long that will last," Turner said.
She said that she had seen casualties of the oil- a bird that had been drifting in the slick for some time and was so weak that it could barely move.
"Its feathers were slippery with oil sheen ... it didn't have long to live."
Around 800,000 litres of oil are spewing from the blown-out well daily into the Gulf of Mexico.
The oil began leaking from the ocean floor after the Deepwater Horizon rig exploded on April 20, killing 11 workers.
Oil had been leaking in three places since the explosion, but BP announced on Wednesday that one small leak had been capped.
The rest of the oil is coming from the blowout preventer at the well, a heavy piece of machinery designed to prevent such a leak.
It is believed to have failed in the April 20 explosion, and crews have been unable to shut it off using robotic devices.
The introduction of the metal container is not expected to solve the problem altogether, and crews are drilling a relief well to take the pressure off the blown-out well at the site, but that could take up to three months to complete.
While similar devices have been used in the past, they have never been tested at such a depth, and it was not clear whether it would function properly.