The Fold will go on sale in just over two months time, earlier than many expected.
The Galaxy S10 5G features the firm's biggest-ever non-folding phone display and promises faster data speeds when networks become available.
The S10 line-up also includes the introduction of a lower-cost model.
Samsung had previously acknowledged that the cost of its S9 range had contributed to "lower-than-expected sales".
Open-up screen
Samsung said the Galaxy Fold would open up to create a 7.3in (18.5cm) tablet-like display and would be able to run up to three apps at once.
A demo showed off "app continuity" features by which the device transferred from one mode to another much more smoothly than had been the case with an earlier foldable phone - Royole's FlexPai.
One example involved a Google Maps screen appearing on the Fold's smaller front display and then expanding to a larger view when the handset was opened following a one-second pause.
Samsung added that Whatsapp, Facebook, YouTube and Microsoft Office would also be optimised to suit the new form-factor.
It said that it had designed a new type of hidden hinge system that would withstand hundreds of thousands of folds and unfolds, and contained a battery on each side to extend its runtime.
A 4G version of the Galaxy Fold is set to go on sale on April 26 and will start at $1,980 (£1,515). A more expensive 5G edition was also promised.
Samsung described it as being a "luxury" item.
"Fold is an experience that gives people who want a phone but also a larger screen with no compromise on the phone experience," commented Carolina Milanesi from the consultancy Creative Strategies.
"There's a lot of tech packed in there. And it makes sense to have kept it under $2,000 even if only for the psychological effect that has."
But another market watcher still had doubts.
"In theory, foldables are hugely attractive: they pack a giant screen into a small design," commented Neil Mawston from the research firm Strategy Analytics.
"But in reality, consumers don't know exactly how they will work, and the applications for them are still fairly immature.
"You can look back at history at the dual-screen folder phones that ZTE and NEC and others release. They haven't sold particularly well mostly due to price and lack of distribution."So, there's good potential, but still a lot of uncertainty."
Analysis:
In the short demonstration we saw today, Samsung's use case of watching entertainment, playing games and app multi-tasking will make a lot of sense for a lot of people. This is a creative feat of engineering. Folding screens seem like a good idea.
What won't work, however, is the price. I've been at many launch events like this, and normally the worst case scenario for the firms putting on the show is a lack of applause when the price is announced. Today we saw something worse - loud grumbles, even some laughter. $1,980? Simply too much.
Also, I wonder about some other aspects of this phone we can't judge yet as we haven't had a chance to hold it. When Samsung's head of mobile placed it into his suit pocket on stage, it landed with all the grace of a cartoon anvil.
So: possibly heavy, with two likely-hot batteries, and a huge price tag.
Creative, sure? Practical? For me, Samsung has fallen short - but the effort should excite gadget fans who have been longing for something different for so long. I suspect this device will have people flocking to stores to see it up close, if not to actually purchase it.
Galaxy S series
The S10 series is likely to remain Samsung's focus when it comes to sales for the foreseeable future.
The S10 and S10+ will cost more than the phones they supersede - beginning at £799 and £899 respectively when they go on sale on 8 March.
But the S10e means the Galaxy S range now starts at a lower price-point - £669 - albeit with lower specifications to match.
The S10 5G is yet to be priced and only has a vague "summer" release planned.
"Having a 5G variant is strategically important for Samsung as it gives them the jump on Apple and helps maintain the firm's brand strength and perceived technology leadership," commented Ben Wood, from the CCS Insight consultancy.
"It also gives the operators a tier-one brand for their 5G launches.
"But as far as consumers are concerned, unless you have a very good reason to buy a 5G phone this summer, one of the other three S10 handsets is probably a better investment, and will be viable for use for many years."
The launch comes days before Mobile World Congress in Barcelona - a trade event where Samsung's rivals will unveil new handsets of their own.
Cut-out cameras
All four versions of the S10 are distinguished from last year's models by embedding the front cameras within their displays.
Samsung refers to this as being the Infinity O design, but it is more commonly referred to as the "hole punch".
The move allows the phones to feature a thinner top bezel without having the kind of "notch" found on many rivals.
It has, however, caused the firm to ditch the eye iris-scanner introduced in the S8.
Samsung says a new ultrasonic fingerprint sensor placed under the screens of the three higher-end phones offers close to the same level of security, and is more convenient to use than a scanner formerly placed on phone backs.
Samsung demoed the facility at a dual London and San Francisco launch as a way to recharge a new pair of Bluetooth headphones without having to use a separate cable or power mat.
Smarter photography
All four devices now feature a 10 megapixel selfie camera and introduce a 16MP "ultra-wide" rear version, which offers a slightly larger field-of-view than our eyes.
four devices now feature a 10 megapixel selfie camera and introduce a 16MP "ultra-wide" rear version, which offers a slightly larger field-of-view than our eyes.
The S10+ also has a second selfie camera to help it take depth readings.
Other improvements include:
- the ability to detect 10 additional types of scene, so as to automatically adjust the colours and sharpness to suit the subject. These now include shoes, cats and clothing
- a "super steady" video mode that combines both digital and optical stabilisation to create the kind of smooth footage that would traditionally have required a gimbal or other add-on stabiliser
- an Instagram mode makes it easier to upload images to the social network directly from Samsung's camera app.