Context

This line is spoken by Ed McMahon when introducing Johnny Carson on The Tonight Show Starring Johnny Carson (1962-1992). 

Before Jimmy Fallon and Jay Leno, The Tonight Show was hosted by another famous J: Johnny Carson.

Carson's sidekick was Ed McMahon. Before he handed out gigantic checks for Publisher's Clearinghouse (oh, wait, that's a myth) and searched for stars on Star Search (like Britney and Bey) he manned the mic as announcer, calling Carson to the stage with a hearty "Heeeere's Johnny!" every night.

Where you've heard it

This quote was popularized by ranting psychopath Jack Torrance (played by Jack Nicholson) When he chops down a door with an axe, he announces himself to his terrified wife by hissing "Heeeeere's Johnny!" making this a catchphrase favored by crazy serial killers across the globe.

Additional Notable References: 

• Weird Al Yankovic has a song called "Here's Johnny."
• The LEGO Shining uses the famous quote. 
• Johnny Carson sued a porta-potty maker who named his johns "Here's Johnny." 

Pretentious Factor

If you were to drop this quote at a dinner party, would you get an in-unison "awww" or would everyone roll their eyes and never invite you back? Here it is, on a scale of 1-10.

While not exactly pretentious, this quote is the sign of an unstable mind. All quoting and no originality makes Jack a crazy person.