Rømer made his discovery regarding the speed of light while working at the Royal Observatory in Paris and studying Jupiter's moon Io.

Rømer was born on 25 September 1644 in Aarhus to merchant and skipper Christen Pedersen (died 1663), and Anna Olufsdatter Storm (c. 1610 – 1690), daughter of a well-to-do alderman.

His mentor at the University was Rasmus Bartholin, who published his discovery of the double refraction of a light ray by Iceland spar (a transparent form of the mineral calcite) in 1668, while Rømer was living in his home.

Rømer was given every opportunity to learn mathematics and astronomy using Tycho Brahe's astronomical observations, as Bartholin had been given the task of preparing them for publication.

[6] Rømer was employed by the French government: Louis XIV made him tutor for the Dauphin, and he also took part in the construction of the magnificent fountains at Versailles.

[10] In 1700, Rømer persuaded the king to introduce the Gregorian calendar in Denmark and Norway – something Tycho Brahe had argued for in vain a hundred years earlier.

After studies in Copenhagen, Rømer joined Jean Picard in 1671 to observe about 140 eclipses of Jupiter's moon Io on the island of Hven at the former location of Tycho Brahe’s observatory of Uraniborg, near Copenhagen, over a period of several months, while in Paris Giovanni Domenico Cassini observed the same eclipses.

[24]Oddly, Cassini seems to have abandoned this reasoning, which Rømer adopted and set about buttressing in an irrefutable manner, using a selected number of observations performed by Picard and himself between 1671 and 1677.

Rømer presented his results to the French Academy of Sciences, and it was summarised soon after by an anonymous reporter in a short paper, Démonstration touchant le mouvement de la lumière trouvé par M. Roemer de l'Académie des sciences, published 7 December 1676 in the Journal des sçavans.

[25] Unfortunately, the reporter, possibly in order to hide his lack of understanding, resorted to cryptic phrasing, obfuscating Rømer's reasoning in the process.

[28] By trial and error, during eight years of observations Rømer worked out how to account for the retardation of light when reckoning the ephemeris of Io.

When the angle α is 180° the delay becomes 22 minutes, which may be interpreted as the time necessary for the light to cross a distance equal to the diameter of the Earth's orbit, H to E.[28] (Actually, Jupiter is not visible from the conjunction point E.) That interpretation makes it possible to calculate the strict result of Rømer's observations: The ratio of the speed of light to the speed with which Earth orbits the sun, which is the ratio of the duration of a year divided by pi as compared to the 22 minutes 365·24·60⁄π·22 ≈ 7,600.

A plaque at the Observatory of Paris, where the Danish astronomer happened to be working, commemorates what was, in effect, the first measurement of a universal quantity made on this planet.

[56] In the 1960s, the comic-book superhero The Flash on a number of occasions would measure his velocity in "Roemers" [sic], in honour of Ole Rømer's "discovery" of the speed of light.

[57][better source needed] In Larry Niven's 1999 novel Rainbow Mars, Ole Rømer is mentioned as having observed Martian life in an alternate history timeline.