The orbit of Mercury has a high eccentricity, with the planet's distance from the Sun ranging from 46 million to 70 million kilometres. Among the major planets, only Pluto has a more eccentric orbit. However, because of the smallness of Mercury's orbit, all of the planets except the Earth and Venus have a larger spread between perihelion and aphelion (Mars' is 42.6 Gm to Mercury's 23.8 Gm, for example). There are even several outer planet satellites that beat Mercury's spread: Saturn's S/2004 S 18 (with 30.8 Gm) and Neptune's Psamathe and S/2002 N 4 (42.0 and 47.9 Gm, respectively).

When it was discovered, the slow Precession of Mercury's orbit around the Sun could not be completely explained by Newtoian mechanics, and for many years it was hypothesized that another planet might exist in an orbit even closer to the Sun to account for this perturbation (other explanations considered included a slight oblateness of the Sun). The hypothetical planet was even named Vulcan. However, in the early 20th century, Albert Einstein's General Theory of Relitivity provided a full explanation for the observed precession. Mercury's precession showed the effects of mass dilation, providing a crucial observational confirmation of Einstein's predictions. This was a very slight effect: the Mercurian relativistic perihelion advance excess is a mere 43 arcseconds per century. The effect is even smaller for the remaining planets, being 8.6 arcseconds per century for Venus, 3.8 for the Earth, and 1.3 for Mars.

Research indicates that the eccentricity of Mercury's orbit varies chaotically from 0 (circular) to a very high 0.47 over millions of years. This is thought to explain Mercury's 3:2 spin-orbit resonance (rather than the more usual 1:1), since this state is more likely to arise during a period of high eccentricity.