Born at Pabsthaus, in Germany, Galle was chief assistant to Johann Encke at the Berlin Observatory at a crucial moment in the history of planetary astronomy. Urbain Leverrier had worked out what he considered to be the position of an as yet undiscovered planet. Having had some contact with Galle, Leverrier wrote to him on 18 September 1846, asking him to try to check his prediction. Galle started observing on 23 September. He was favored by having an unpublished copy of a new star chart covering the right part of the sky and, aided by Louis D'Arrest, he found a star that was not on the chart. A wait of 24 hours showed that it had moved against the background of the fixed stars and so was a planet – it was the planet Neptune.

Galle also made an important contribution to determine the mean distance of the Sun from the Earth (the astronomical unit or AU). Conventional means of determining the AU had leaned heavily on the two transits of Venus in each century. In practice it turned out to be difficult to measure accurately the moment of first contact. Galle proposed instead, in 1872, that measuring the parallax of the planetoids would give a more reliable figure. (Harold Spencer Jones (1890–1960) followed this procedure in 1931 when the planetoid Eros came within 16 million miles of the Earth. He was able to calculate the AU to within 10,000 miles.) In 1851 Galle became director of the Breslau Observatory. He lived long enough to receive the congratulations of the astronomical world on the 50th anniversary of the discovery of Neptune in 1896.