He enjoyed playing with science-themed toys while a child, and later mowed lawns as a teenager so that he might purchase a telescope in high school.

He was initially undecided about pursuing a career in astronomy after his undergraduate education, but made up his mind after seeing the images of Jupiter sent back to Earth by Voyager 2.

[6][7] In 2003, Showalter and Jack J. Lissauer discovered two new moons of Uranus (Mab and Cupid) in Hubble Space Telescope images.

A search for faint dust rings of Pluto using the Hubble Space Telescope in 2011 led to the discovery of the fourth moon Kerberos.

[16][17] On July 15, 2013, a team of astronomers led by Showalter discovered a previously unknown fourteenth moon of Neptune in images taken by the Hubble Space Telescope from 2004 to 2009.