In June 2000, a few months after Viacom (which CBS founded in 1952 as television syndication distributor CBS Films, Inc., and later spun off in 1971 after the then-recently implemented Financial Interest and Syndication Rules barred networks from holding financial interest in syndicated programming content) completed its $37 billion merger with CBS Corporation, CBS reached an agreement with new corporate sister Nickelodeon to air programming from its Nick Jr. block beginning that September.
The new block, Nick Jr. on CBS debuted on September 16, 2000, replacing the Nelvana-produced animation block CBS Kidshow, which ended its run the week prior on September 9. For the first two years of the Nickelodeon agreement, the block exclusively aired preschool-targeted programming from Nick Jr., including interstitials from the Nickelodeon block's animated mascot, Face. On September 14, 2002, the block was rebranded from Nick Jr. on CBS to simply Nick on CBS; at that time, its programming content expanded to include live-action and animated Nickelodeon series aimed at children between 7 and 16 years of age, in addition to the Nick Jr. series.
As with the predecessor CBS Kidshow and Think CBS Kids blocks, all of the programs within the block complied with educational programming requirements defined by the Children's Television Act, although the educational content in some of the programs was tenuous in nature. It was partly for this reason why some of Nickelodeon's most popular programs (most notably, SpongeBob SquarePants – at the time the cable channel's most popular series – which only aired on CBS as one-time special presentations and not as a regular part of the block's lineup) were not included as part of the CBS block, even during the more open-formatted Nick on CBS era. Rugrats also aired as one-time special presentations, before it was added as a regular series within the block.
The older-skewing Nickelodeon series were removed from the block on September 18, 2004, refocusing the block back exclusively toward preschooler-oriented series; the block also began incorporating interstitial hosted segments featuring Piper O'Possum. Fifteen months later on December 31, 2005, Viacom formally split into two separate companies under the shared control of National Amusements (owned by Sumner Redstone), with CBS and all related broadcasting, television production and distribution properties as well as some non-production entities becoming part of the standalone company CBS Corporation, while Nickelodeon and its parent subsidiary MTV Networks became part of a new company under the Viacom name.
Less than a month later on January 19, 2006, CBS announced that it would enter into a three-year programming partnership with DIC Entertainment to produce a new children's program block for the three-hour Saturday morning time slot featuring new and recent series from its program library.Nick Jr. on CBS ended its run on September 9, 2006 and was replaced the following week (on September 16) by the KOL Secret Slumber Party on CBS.
^CBS aired the SpongeBob episodes, "Christmas Who?", on Christmas 2002 and "The Sponge Who Could Fly" on Earth Day 2003 during the block's run; three other SpongeBob specials ("Friend or Foe" on June 15, 2007 and "WhoBob WhatPants" on Earth Day 2009, and "It's a SpongeBob Christmas!" on November 23, 2012) aired after Nick Jr. on CBS/Nick on CBS ended its run.