He is also known for his recurring television roles in Law & Order: SVU (2009–2012), The Good Wife (2013–2014), Madam Secretary (2014) and Will & Grace (2018–2019).

[8] In a piece he wrote for the improvisational troupe's book, Klein recalled sitting in a room full of other hopefuls, including Fred Willard.

When he returned to New York City a year later, he was cast by Mike Nichols in the Broadway musical The Apple Tree.

[9] Klein's first major appearance was as host of the 1970 summer replacement television series Comedy Tonight, on which were introduced many of the routines that in the next few years would be released on record albums.

[10] Klein starred in HBO's first stand-up comedy special in 1975 during the cable channel's early broadcast days and has continued to appear in several more one-man shows which have typically concluded with his "I can't stop my leg" routine.

[12][13] His skits included "Tough Director" in 1975,[14] "Nick The Lounge Singer Sings Star Wars Theme" in 1978[15] and "The Olympia Restaurant: Cheeseburger, Chips and Pepsi" in 1978.

[20] The following year, Klein hosted Monty Python Live at Aspen, a reunion and tribute show for the five surviving members of the British comedy troupe, in a special that appeared on HBO in 1998.

He also goes into other things that he has observed in his life, such as substitute teaching, 1970s FM radio disc jockeys, late-night delicatessens, and annoying commercials (e.g., Geritol).

"[citation needed] His next album, Mind Over Matter (1974), included extensive discussion of the Watergate scandal and another song—the title track—about a kid who turned to humor to become popular.

[23][deprecated source] "The funniest album by a standup comic since George Carlin's Class Clown leaves behind the grammar-school nostalgia—which although frequently amusing always seemed formulaic when it wasn't—that kept Klein from sounding commercially uncompromised.

Klein's follow-up album, New Teeth (1975, Epic/CBS Records), featured the comedian's on-stage work on tracks such as "Mother Isn't Always Right" and his transposition of George Carlin's "Seven Words You Can't Say On Television," titled "Six Clean Words You Can Say Anywhere," with studio-recorded material such as "Continental Steel" and "On the Bayou.

Various comedians including Bill Maher, Billy Crystal, Richard Lewis, and Jay Leno, all have also cited Klein as a comedic influence.