Both the title single and the album rose to the top of their respective US charts (the Billboard Hot 100 singles chart and the Billboard 200 albums chart). The title song and its follow-up, "Please Mr. Please", were both top 10 on three Billboard charts: the Hot 100, Adult Contemporary, and Country.
The title song was Newton-John's first charting single in Japan, where it reached number 26 on the Oricon singles chart.
Rolling Stone noted that "with the huge success of Have You Never Been Mellow, Olivia Newton-John joins Helen Reddy and Karen Carpenter as one of the decade's three white female MOR superstars...Newton-John's records combine standard MOR production with instrumentation borrowed from country music, and Newton-John, who is British born, affects a country-girl personality convincingly enough to sell to the country as well as the pop market. Her voice is very pretty, especially in the upper register... She looks and sounds like a breathlessly innocent real-life doll. The smash title cut of her new album is its most ingenuous. Three other songs — "It's So Easy," "Please Mr. Please" and Rick Nelson's "Lifestream"—are possible follow-ups."[5]
Billboard praised the album, noting that "last year's new superstar shows no signs of slowing down this mix of country, soft rock and easy listening cuts designed to appeal to the fans she has garnered in all three areas. Miss Newton-John has developed into an excellent singer, she does not go out of her range and stands as a remarkable example of musical versatility. Backed by fine production and arrangements throughout, the set is basically an extension and progression of her last effort...Few flaws here and an almost certain bet to strap her firmly in the superstar category for good." [6]
Cashbox stated that "Olivia Newton-John's latest LP is another much expected treat. The Grammy winner is at her best here singing a collection of songs that display the charm, poise and total command she exercises everytime she sings."[7]
AllMusic called the title track "a masterpiece of songwriting by her producer, John Farrar, an exquisitely pleasant melody with a sunny and peaceful theme." In their review of the entire album, critic Joe Viglione
noted that "from beginning to end Have You Never Been Mellow is Olivia Newton-John showing why she could hold her own on the charts with as powerful a star as Elton John. Recorded at EMI Studios, London, it has impeccable sound and a special vibe."[8]