With this season, The Simpsons established itself as the longest-running American primetime television series surpassing Gunsmoke.
The season received mainly positive reviews from critics, with many praising "The Squirt and the Whale", "To Surveil with Love" and "The Bob Next Door". The show moved up 16 positions in the Nielsen ratings from the previous season and received numerous award nominations, winning two — an Emmy Award for Anne Hathaway for her voicing in "Once Upon a Time in Springfield", and an Annie Award for "Treehouse of Horror XX".
The season featured eight holdover episodes from the season 20 (LABF) production line. John Frink was credited as an executive producer for the first time.
In 2009, to celebrate the 20th anniversary of the premiere of The Simpsons, Fox announced that a year-long celebration of the show titled "Best. 20 Years. Ever." would run from January 14, 2009 to January 14, 2010.
As part of the celebration, documentary filmmaker Morgan Spurlock directed and produced The Simpsons 20th Anniversary Special – In 3-D! On Ice!, a documentary special that examined the "cultural phenomenon of The Simpsons". Despite the title, Spurlock said the special "most likely not be in 3-D nor on ice." Production began at Comic-Con 2009, and the show aired on January 10, 2010 on Fox. It included interviews with the cast and fans of the show.