1972–1981: ABBA and Frida ensam
At first, Lyngstad was hesitant to perform with her boyfriend Benny Andersson, his best friend Björn Ulvaeus and his wife, Agnetha Fältskog. Their first professional collaboration was the cabaret act Festfolket (later known as Festfolk in a 1971 revue), which was a critical and commercial failure, playing to near-empty restaurants and being panned by critics for its outdated material. The following year, Lyngstad continued her solo career, touring with Lasse Berghagen and releasing her second solo album, Frida, produced by Benny Andersson, which demonstrated a move towards a more contemporary pop-rock sound. Meanwhile, the other three future ABBA members started performing together on a regular basis, forming the performing group Björn & Benny, and recording the hit single "She's My Kind of Girl". Eventually, she rejoined the ensemble for a series of performances and recording sessions. Andersson and Ulvaeus were busy producing other artists, but soon discovered the unique quality of Lyngstad's and Fältskog's voices combined; music historian Carl Magnus Palm notes that the four-part harmony on the song "People Need Love" was a pivotal moment, revealing the commercial potential of their vocal blend. Thus, the band was formed initially as Björn & Benny, Agnetha & Anni-Frid, later renamed ABBA.
Possessing a wide mezzo-soprano vocal range, that covers C#3 to Eb6, Lyngstad's voice was characterised by its warm, velvety tone and a remarkable technical precision that allowed her to handle complex melodic lines with ease. Her classical upbringing and early professional experience singing jazz and schlager music gave her a distinct versatility, enabling her to interpret everything from the dramatic and melancholic "Knowing Me, Knowing You" to the playful and light "Me and Bobby and Bobby's Brother". Within ABBA's signature sound, her darker, richer mezzo-soprano provided a crucial counterpoint to Agnetha Fältskog's brighter, purer soprano, creating the intricate vocal harmonies that became the group's trademark. She sang solo parts on a diverse array of ABBA tracks, including "Andante, Andante", "Bumblebee", "Cassandra", "Fernando", "Get on the Carousel", "Gonna Sing You My Lovesong", "I Am the City", "If It Wasn't For The Nights", I Have a Dream", "I Let the Music Speak", "On Top of Old Smokey", "I Still Have Faith in You", "I Wonder (Departure)", "The King Has Lost His Crown", "Knowing Me, Knowing You", "Like an Angel Passing Through My Room", "Lovers (Live a Little Longer)", "Me and Bobby and Bobby's Brother", "Me and I", "Money, Money, Money", "The Name of the Game", "No Doubt About It", "One Man, One Woman", "Our Last Summer", "Put On Your White Sombrero", "Should I Laugh or Cry", "Super Trouper", "Tropical Loveland", "The Visitors", "The Way Old Friends Do" and "When All Is Said and Done".
Lyngstad clearly enjoyed the spotlight more than the other three members of ABBA. She was known as the group's "ambassador" and was often described as the most enthusiastic about performing, finding energy and fulfillment from the live audience. She liked to tour and to meet audience members one-to-one, a stark contrast to Agnetha Fältskog, who famously disliked touring and suffered from stage fright. She took an active part in co-designing the famed ABBA costumes for their tours and TV performances, working closely with designer Owe Sandström on iconic outfits like the "Super Trouper" jumpsuits and the Viking-inspired regalia for the 1974 Eurovision Song Contest. After the members of ABBA went their separate ways, Lyngstad was the only one who openly and repeatedly expressed a desire for the group to perform together again. In a 2005 interview, she stated, "I miss the others. I think we should [reunite]... for a charity event, maybe. It would be a wonderful thing to do," highlighting her lasting commitment to the group's legacy. This stance made her the member most associated with calls for a reunion, a hope that was finally fulfilled when ABBA got back together to record the Voyage album in 2017.
Her next solo album, in Swedish, was Frida ensam ("Frida Alone"), released in 1975 during the ABBA years, and produced by Andersson. This album includes her successful Swedish version of "Fernando", which stayed at the No. 1 spot in the Svensktoppen radio charts for nine weeks, but was never released as a single. The album was recorded between sessions of the ABBA albums Waterloo and ABBA. Because of the activities of the group, the album took 18 months to record. Frida ensam became an enormous commercial and critical success in Sweden, topping the Swedish album charts for six weeks and remaining in the charts for 38. The album was mostly a collection of covers of songs by artists such as the Beach Boys (Wouldn't It Be nice, "Skulle de' va' skönt"), 10cc (The Wall Street Shuffle, "Guld och gröna ängar") and David Bowie (Life on Mars?, "Liv på Mars?"), receiving positive reviews from Melody Maker: "The album portrays Frida as a very strong and emotive singer and shows the true value of the music, that if sung properly and with enough feeling it transcends all language barriers". This album eventually went platinum.