She suffered from anorexia nervosa, a little known disease at the time, and died at the age of 32 from heart failure, later attributed to complications related to her illness.

Carpenter was born in New Haven, Connecticut, to Agnes Reuwer Tatum and Harold Bertram Carpenter. When she was young, she enjoyed playing baseball with other children on the street. On the TV program, This Is Your Life, Carpenter stated that she liked pitching. In the early 1970s, she went on to play as the pitcher on the Carpenters' official softball team.

Karen's brother, Richard, had developed an interest in music at an early age, becoming a piano prodigy. Karen showed less interest in music as a young child. The family moved in June 1963 to the Los Angeles suburb of Downey.

When Karen entered Downey High School, she joined the school band. The conductor (who had previously taught her older brother) gave her the glockenspiel, an instrument she disliked. After admiring the performance of a friend named Frankie Chavez, she asked the conductor if she could play the drums instead. Drumming came naturally to Carpenter, and she practiced for several hours a day. She and Richard made their first recordings in 1965 and 1966. The following year, Karen began dieting. Under a doctor's guidance Karen, who stood 5'5" and weighed 145 pounds, went on the Stillman Diet. She rigorously ate lean foods, drank 8 glasses of water a day, and avoided fatty foods. Eventually, Karen dropped to 120 pounds. She would stay at 120 pounds until 1974.

From 1965 to 1968, Karen, her brother Richard and his college friend Wes Jacobs, a bassist and tuba player, formed The Richard Carpenter Trio. The band played jazz at numerous nightclubs, and also appeared on a TV talent show called Your All American College Show.

Karen, Richard, and other musicians, including Gary Sims and John Bettis, also performed as an ensemble known as Spectrum. Spectrum focused on a harmonious, vocal sound, and recorded many demo tapes in the garage studio of friend and bassist Joe Osborn. Many of those tapes were rejected. According to former Carpenters member John Bettis, those rejections "took their toll."

Finally, in April 1969 A&M Records signed the Carpenters to a recording contract. Karen Carpenter sang most of the songs on the band's first album, "Offering" (later retitled Ticket to Ride). The issued single (later the title track), which was a cover of a Beatles song, became their first single: it reached #54 on the Billboard Hot 100 Charts. Their next album, 1970's Close to You, featured two massive hit singles: "(They Long to Be) Close to You" and "We've Only Just Begun." They peaked at #1 and #2, respectively, on the Hot 100.

Karen Carpenter started out as both the group's drummer and lead singer, and she originally sang all her vocals from behind the drum set. Eventually, she was persuaded to stand at the microphone to sing the band's hits while another musician played the drums, although she still did some drumming. (Former Mouseketeer Cubby O'Brien served as the band's other drummer for many years.)

After the release of Now & Then in 1973, the albums tended to have Karen singing more and drumming less. Karen rarely selected the songs she would sing, and often felt she had very little control over her life. She dieted obsessively and developed anorexia nervosa. At the same time, her brother Richard developed an addiction to Quaaludes.

The Carpenters frequently cancelled tour dates, and they stopped touring altogether after their September 4, 1978 concert at the MGM Grand in Las Vegas. In 1981, after the release of the Made in America album (which turned out to be their last) the Carpenters returned to the stage and did some tour dates, including their final live performance in Brazil.