He made the solo descent in a submarine called Deepsea Challenger, taking over two hours to reach the bottom.
He spent more than four hours exploring the ocean floor, before a speedy ascent back to the surface.
His craft was kitted out with cameras so he could film the deep in 3D.
"It was absolutely the most remote, isolated place on the planet," Mr Cameron told BBC News.
"I really feel like in one day I've been to another planet and come back."
This is only the second manned expedition to the ocean's deepest depths - the first took place in 1960 when US Navy Lt Don Walsh and Swiss oceanographer Jacques Piccard spent about 20 minutes on the ocean floor in a bathyscaphe called the Trieste.
Lt Walsh, who is now in his 80s, joined Mr Cameron and his team of engineers out at sea for the dive.
"It did bring back a lot of memories, just being out there and remembering what we did there," he told BBC News. "It was really grand."
Director James Cameron resurfaced after spending four hours on the ocean floor
Mr Cameron has spent the past few years working in secret with his team of engineers to design and build the craft, which weighs 11 tonnes and is more than 7m (23ft) long.
He describes it as a "vertical torpedo" that slices through the water allowing him a speedy descent.
The extraordinary attention to detail prevented him from suffering from too much nervousness.
"I can't say that I wasn't apprehensive in the last few days and even the weeks leading up to this, but there's another part of my mind that really understands the engineering and knows why we did everything the way we did," he said.
"Any apprehension I had I left at the hatch. When I went into the sub, I was all pilot at that point."
The tiny compartment that the film-maker sits in is made from thick steel, which is able to resist the 1,000 atmospheres of pressure he experienced at full ocean depth.
The rest of the vertical column is made from a material called syntactic foam - a solid made mostly of hollow "microballoons" - giving it enough buoyancy to float back up.
The sub has so many lights and cameras that it is like an underwater TV studio - with Mr Cameron able to direct and film the action from within. He intends to release a documentary.
It also has robotic arms, allowing him to collect samples of rocks and soils, and a team of researchers are working alongside the director to identify any new species. He says that science is key to his mission.
But the first task was to get to the inky depths - which despite untold hours of training, still surprised Mr Cameron.
"My reference frame was going to the Titanic 10 or 12 years ago, and thinking that was the deepest place I could ever imagine," he recalled.
"On this dive I blazed past Titanic depth at 12,000 ft and was only a third of the way down, and the numbers keep going up and up and up on the depth gauge.
"You just kind of look at them with a sense of disbelief, and you wonder if the bottom is ever going to be there."
At the bottom, Mr Cameron encountered incredibly fine silt, which he had to be careful not to disturb. He said he spotted a few small, as-yet unidentified life forms but found the depths to be a "sterile, almost desert-like place".
Other teams, such as Scotland's Oceanlab, have also been dropping simple landers loaded with bait and cameras into the deepest ocean.
While places like the Mariana Trench were once thought to be of little interest, there has been a recent resurgence of scientific interest in the deep.
Scientists are finding life that can resist the colossal pressures, from deep-sea fish to shrimp-like scavengers called amphipods, some of which can reach 30cm (1ft) long.
They are also trying to understand the role that deep seas trenches play in earthquakes - these cracks in the sea floor are formed at the boundary of two tectonic plates and some believe the push and pull taking place deep underwater could be the cause of major earthquakes, such as the 2011 quake that resulted in such devastation in Japan.
But some scientists question whether manned exploration provides the best platform for scientific research.
Dr Alan Jamieson, from Oceanlab, said: "I think what James Cameron has done is a really good achievement in terms of human endeavour and technology.
"But my feeling is that manned submersibles like this are limited in scientific capabilities when compared to other systems, mostly due to the fact there is someone in it. Remote or autonomous systems can collect a far greater volume of useful scientific data for far less money."
His craft may also soon be joined by other manned submersibles vying to reach the ocean's deepest depths.
One of these crafts, the DeepFlight Challenger, belongs to former real estate investor Chris Welsh, and is backed by Virgin's Richard Branson. It is about to begin its water trials.
Its design is based on a plane, and Mr Welsh says he will be "flying" down to the deepest ocean.
Google's Eric Schmidt has helped to finance another sub being built by a US marine technology company called Doer Marine. They want this sub to carry two to three people, and are placing a heavy emphasis on science.
And Triton submarines, a Florida-based submersible company, intends to build a sub with a giant glass sphere at its centrepiece to take tourists down to the deepest ocean for $250,000 a ticket.
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