Answer by taking a quote from the following article:

Rowan & Martin's Laugh-In (often simply referred to as Laugh-In) is an American sketch comedy television program that ran for 140 episodes from January 22, 1968, to March 12, 1973, on the NBC television network. It was hosted by comedians Dan Rowan and Dick Martin. Laugh-In originally aired as a one-time special on September 9, 1967, and was such a success that it was brought back as a series, replacing The Man from U.N.C.L.E. on Mondays at 8 pm (ET). The title of the show was a play on the "love-ins" or "be-ins" of the 1960s hippie culture, terms that were, in turn, derived from "sit-ins", common in protests associated with civil rights and antiwar demonstrations of the time.

The 1970-71 season brought new additions to the cast include tall, lanky, sad-eyed Dennis Allen, who alternately played quietly zany characters and straight man for anybody's jokes; comic actress Ann Elder, who also contributed to scripts, tap dancer Barbara Sharma, and Johnny Brown.  Arte Johnson, who created many memorable characters, insisted on star billing, apart from the rest of the cast. The producer mollified him, but had announcer Gary Owens read Johnson's credit as a separate sentence: "Starring Dan Rowan and Dick Martin! And Arte Johnson! With Ruth Buzzi ..." This maneuver gave Johnson star billing, but made it sound like he was still part of the ensemble cast. Johnson left the show after the 1970-71 season. Henry Gibson also departed after the 1970-71 season. Johnson and he were replaced by former Hogan's Heroes stars Richard Dawson and Larry Hovis, both of whom had appeared occasionally in the first season. However, the loss of Johnson's many popular characters caused ratings to drop further.  After winning the Academy Award for Best Supporting Actress in Cactus Flower, Goldie Hawn made a guest appearance in the third episode of the fourth season. She began the episode as an arrogant snob of an actress; however, a bucket of water thrown at her transformed her back to her giggling dumb blonde persona.  The show celebrated its 100th episode during the 1971-72 season, with Carne, Worley, Johnson, Gibson, Graves, and Tiny Tim all returning for the festivities. John Wayne was on hand for his first cameo appearance since 1968.

Did anyone else leave?
Johnson and he were replaced by former Hogan's Heroes stars Richard Dawson and Larry Hovis,