Kaler started his professional careeer with appointments as a research and teaching assistant at the University of Michigan from 1958 to summer 1960. In 1961 he worked as an astronomer with the United States Naval Observatory. In 1964 he was appointed as an assistant professor of Astronomy by the University of Illinois, and promoted to associate professor in 1968 and to a full professor position in 1976 (all at University of Illinois). Since 1995 he is Campus Honors Faculty. In 2003 he retired to become professor emeritus at the University of Illinois.

Kaler published over 120 papers. Examples include work on

• the chemical composition of planetary nebulae including their electron densities, on
• emission lines of planetary and diffuse nebulae, on
• the development of shells in planetary nebulae, and on
• stellar evolution.

He has held Fulbright and Guggenheim Fellowships, and has been awarded medals for his work from the University of Liège in Belgium and the University of Mexico. He gave the Armand Spitz Lecture to the Great Lakes Planetarium Association and the Margaret Noble Address to the Middle Atlantic Planetarium Society. In 2003 he received the 2003 Campus Award for Excellence in Public Engagement by the University of Illinois.

Jim Kaler has written for a variety of magazines, and was a consultant for Time-Life Books. He has long appeared on Illinois television and radio. In addition to two textbooks and three audio courses, he published several books, including

• Stars and their Spectra,
• The Ever-Changing Sky,
• Extreme Stars (American Association of Publishers Outstanding Professional and Scholarly Title in Physics and Astronomy for 2001),
• The Cambridge Encyclopedia of Stars,
• Stars and Cosmic Clouds,
• The Little Book of Stars,
• The Greatest Hundred Stars, and
• Heaven's Touch (selected Book of the Week by Times Higher Education in September 2009).

He has served as President of the Board of Directors of the Astronomical Society of the Pacific, and of the Board of the Champaign Urbana Symphony Orchestra.

Asteroid 1998 JK was named 17853 Kaler in honor of his outreach activities, and in 2008 he received the American Astronomical Society's Education Prize.

His science WWW-star database " STARS" has scored more than 3 million visitors since its release.