This is a list of notable events, premieres, releases, series cancellations and endings, and other business ventures for Nickelodeon in the year 1999.
- Nick GaS debuts as its own channel.
- Henry & June's Nicktoons Summer Jam replaces Nick in the Afternoon as the annual summer weekday afternoon block.
- January 4 - Franklin
- January 17 - The Brothers Flub
- February 11 - Maisy
- May 1 - SpongeBob SquarePants premieres after the Kids' Choice Awards; it would begin regularly airing on July 17th.
- August 16 - Rocket Power
- October 16 - 100 Deeds for Eddie McDowd and The Amanda Show
- November 28 - Little Bill
- December - Dora & Diego's (Test Pilot)
- December 31 - ChalkZone premieres its first episode as part of the Nick New Year's block, but the rest of the first season would be delayed to 2002.
- The Off-Beats' Valentine's
- The Henry & June Show
- February 20 - Nick GaS ends in preparation for its channel's launch.
- March 27 - Nicktoons TV airs its final broadcast; it would become its own channel in 2002.
- June - Henry & June's Nicktoons Summer Jam
- July 7 - Nick Flicks movie block begins.
- September - 101% Whizbang! With Henry and June
- November - U-Pick Fridays
- February 28 - The Rugrats Movie (Game Boy Color)
- June 29 - Rugrats: Scavenger Hunt
- November 10 - Rugrats: Studio Tour
- March 1 - Aliens
- May 1 - Summer Vacation
- June 1
- Tommy's Last Stand
- July 1 - Cat's Big Night / Dog Behind Bars
- August - CatDog's Big Idea
- September 1
- October 1
- November 1
Home video releases
- March 30
- CatDog: Together Forever
- CatDog: CatDog vs. the Greasers
- The Rugrats Movie is released on VHS, Laserdisc, and DVD
- August 3 - Rugrats: Runaway Reptar
- October 1 - Rugrats: Make Room for Dil
- January 1 - Dora the Explorer season 1, 2, & 3
- March 30 - Nickelodeon enters the DVD market with Paramount Home Video's DVD release of The Rugrats Movie
- February 10 - Tiffany Espensen is born
- May 25 - Brec Bassinger is born
- November 1 - Buddy Handleson is born
- November 11 - Mary Kay Bergman (the original voice of Timmy Turner) dies
Nickelodeon character debuts
- Oh Yeah! Cartoons: Penny Sanchez, Tootie, Jorgen Von Strangle
- SpongeBob SquarePants: SpongeBob SquarePants, Patrick Star, Squidward Tentacles, Mr. Krabs, Sandy Cheeks, Sheldon Plankton, Mermaid Man, Barnacle Boy, Gary the Snail,