Actors including Naya Rivera, Amber Riley and Jane Lynch headed to a bash at the city's Chateau Marmont hotel to celebrate the landmark while the episode aired across the U.S.
The cast and crew gathered inside the venue to toast the achievement and give speeches reminiscing about their early memories of the show, which began in 2008.
Michele wrote in a post on Twitter.com, "Can't wait for tonight's 100th episode of Glee! I'm so proud to be part of such an amazing show that brings me such joy every single day."
She then posted a number of nostalgic pictures including her first day playing character Rachel and another showing her hugging late co-star Cory Monteith underneath a Glee billboard in 2009.
Agron also tweeted, "I can remember the first day like it was yesterday. (Michele), (Rivera) & me in a bathroom with a lot of hairspray."
A number of former cast members returned for the two-part 100th episode, including Kristin Chenoweth and Gwyneth Paltrow.