"What this spacecraft and this team accomplished is unprecedented", said Alan Stern, the mission's principal investigator, before unveiling the first images.
The two-balled shape reminded others of BB-8, the plucky droid from "Star Wars: The Force Awakens".
This are only the first images from the probe, captured at a resolution of about 140 meters per pixel.
"That bowling pin is gone - it´s a snowman if anything at all", Stern said during a NASA briefing.
"That image is so 2018".More news: Tesla cuts prices as 4Q deliveries miss analysts' estimates
An analysis of the picture showed that Ultima's brightest spots reflect 13 percent of the light that falls on them, while the darkest spots reflect only 6 percent.
"We were basically chasing it down in the dark at 32,000 miles per hour". This image was taken by the craft's Multispectral Visible Imaging Camera (MVIC), which combines light from the infrared, red and blue channels.
New Horizons - which previously revealed the majesty of Pluto in 2015 - swooped just 2,200 miles from the surface of MU69, which has been temporarily nicknamed Ultima Thule (the International Association of Astronomers will eventually endow MU69 with its official name). Mission scientists are working on additional funding. Tholins are also behind the reddish color in a crater on the surface of Pluto's biggest moon, Charon, but scientists say the process behind its deposition on Ultima is likely to be different.
The New Horizons team has also been able to determine a rough calculation for 2014 MU69's rotation period at 15 hours +/- 1 hour, meaning that more than half of the object was in daylight as New Horizons passed and was therefore observed. The new images and data confirmed the icy space rock's classification as a contact binary.
The larger sphere is "Ultima" and measures 19 kilometres across. Unlike comets and other objects that have been altered by the sun over time, Ultima Thule is in its pure, original state: It's been in the deep-freeze Kuiper Belt on the fringes of our solar system from the beginning. Then the balls would have been gently drawn together by their mutual gravitational attraction, he said. Another possibility is Ultima could be two objects orbiting each other.
Carly Howett, New Horizons co-investigator, said: "We can definitively say that Ultima Thule is red". He called New Horizons "a time machine", capable of taking scientists back to the moment of our origins.More news: Ball drops in Times Square as thousands ring in the New Year
Future images sent back to earth will be of a higher quality as they will have been taken closer to the object and will benefit from better sunlight.
But Ultima is a further 1.5 billion km further out.
Ultima Thule in colour. This is something of a mystery, because Ultima Thule is thought to be made mostly of ice.
Mr Stern added: "It's going to just get better and better".
During his science presentation, Stern also referred to the two lobes of the space rock as "Ultima" (the larger lobe) and "Thule" (the smaller one). The complete data set collected by New Horizons during the flyby will take 20 months to beam back to NASA scientists, but the initial trickle began with some very blurry photos received Tuesday.