The Nottinghamshire swimmer started the Ocean's 7 Challenge in 2008 with a swim across the English Channel.
He has since conquered seas in Gibraltar, Hawaii, New Zealand, USA and Japan.
The final leg of his challenge saw him swim from Northern Ireland to the west coast of Scotland.
He completed the 21 mile (33km) swim on Wednesday.
Mr Walker said it was a great feeling to finally complete all of the swims, during which he had been stung by jellyfish, encountered sharks and been protected by dolphins.
The 35-year-old has been raising money for marine charities.
During the swim across the Molokai Channel in Hawaii, Mr Walker was stung in the face by several jellyfish and "nearly shot out of the water" after being stung by a Portuguese man o'war.
He also came close to a large tiger shark during the 17-hour challenge.
He said: "I was stung after 14 hours and I can only describe it like someone had slashed me repeatedly across the front of my stomach and down the side.
"I was shouting in agony for around two minutes and then realised I had a 5in (12cm) tentacle stuck to my stomach, which I ripped off."
Whilst swimming the Cook Strait in New Zealand, Mr Walker said he was lucky enough to swim with a pod of dolphins.
He said he also spotted a large shark swimming beneath him and thought the dolphins had come to protect him.
Mr Walker described it as "a real privilege".
As well as seeing numerous sharks during his swims, Mr Walker has also swam close to a pilot whale in the Straits of Gibraltar.
He said the sea temperature on the latest leg across the North Channel was the coldest of the lot.
Swimmer Adam Walker has become the first Briton to complete a series of open-ocean swims across some of the most gruelling channels in the world.