Born in Bolton, he made his fortune as a beer brewer, which enabled him to indulge his interest in astronomy. He built an observatory near Liverpool with a reflector telescope, for which he pioneered the use of an equatorial mount for easy tracking of objects as the earth rotates. He ground and polished the mirror himself, using equipment he constructed.
In 1846 Lassell discovered Triton, the largest moon of Neptune, just 17 days after the discovery of Neptune itself by German astronomer Johann Gottfried Galle. In 1848 he independently co-discovered Hyperion, a moon of Saturn. In 1851 he discovered Ariel and Umbriel, two new moons of Uranus.
In 1855, he built a telescope, which he installed in Malta because of the better observing conditions compared to England.
He won the Gold Medal of the Royal Astronomical Society in 1849, and served as its president for two years starting in 1870.