With the airing of its 20th season, beginning 25 September 2009, "L&O" tied the record held by "Gunsmoke" (1955) for the longest running prime-time drama series on US television (other shows, such as "Saturday Night Live" (1975) and "The Simpsons" (1989) have lasted longer, but in other genres). However, it was announced in May 2010 that the series was not renewed for the 2010-2011 season, so the "Gunsmoke" tie remains unbroken.
The last of the original cast members was Chris Noth (Mike Logan). Although often touted as an original cast member, Steven Hill (Adam Schiff) joined the show after the first episode.
Detective Mike Logan's brown leather coat was not supplied by the wardrobe department. Actor Chris Noth actually bought that coat from a second-hand clothing store.
After her departure from the series, Jill Hennessy wrote and directed the film The Acting Class (2000) using Law & Order's sets. Many L&O cast members such as Jerry Orbach and Angie Harmon volunteered to appear in her film, which was a mock-documentary.
Michael Moriarty resigned at the end of the fourth season after a long, vocal battle of words with Attorney General Janet Reno, who was making efforts to censor television violence. He felt that NBC was trying to silence him when two talk show appearances on the network were pulled at the last moment and his role was reduced considerably in the fourth season episode "Mayhem." Dick Wolf claims this was entirely coincidental. Moriarty claims he was forced into a situation where he had to resign. His character, EADA Ben Stone, also resigned on the show. After quitting the series, Moriarty moved to Canada, where he considered forming a political party.
Jerry Orbach left the show after 12 years to star in the spin-off "Law & Order: Trial by Jury" (2005). He died after filming only two episodes.
Both Steven Hill and Sam Waterston were 68 years old when they started playing the District Attorney on the show. (Steven Hill turned 68 in 1990 and Sam Waterston turned 68 in 2008.)
J.K. Simmons' character, Dr. Emil Skoda, was named after the founder of the Czech engineering and manufacturing firm, Skoda, after which the car company of the same name was eventually renamed.
Dann Florek and Richard Brooks were both let go after the third season because producers felt the show needed female characters. They were replaced by S. Epatha Merkerson and Jill Hennessy respectively. Dann Florek was brought back for "Law & Order: Special Victims Unit" (1999) and Richard Brooks returned in a few episodes of Law & Order to play the same character but as a Defense Attorney.
Season 8 (1997-98) marks the first season of the series in which there were no main character changes. From season 2 on, there was at least one change in the main characters either at the beginning of or during the season.