Formed in 1966 in Chicago, Illinois, USA, Chicago was a consistent hit-making group throughout the 70s and 80s. The band was initially called the Missing Links, next becoming the Big Thing and then, the same year, Chicago Transit Authority, at the suggestion of manager Jim Guercio. The original line-up was Terry Kath (b. 31 January 1946, Chicago, USA, d. 23 January 1978; guitar, vocals), Peter Cetera (b. 13 September 1944, Chicago, Illinois, USA; bass, vocals), Robert Lamm (b. 13 October 1944, Brooklyn, New York, USA; keyboards, vocals), Walter Parazaider (Walt Perry) (b. 14 March 1945, Chicago, USA; saxophone), Danny Seraphine (b. 28 August 1948, Chicago, USA; drums), James Pankow (b. 20 August 1947, Chicago, USA; trombone) and Lee Loughnane (b. 21 October 1941, Chicago, USA; trumpet). The horn section set the group apart from other mid-60s rock bands, although Chicago Transit Authority was preceded on record by similar-sounding groups such as Blood, Sweat And Tears and the Electric Flag.

The band was formed when a group of DePaul University music students began playing a series of late-night jams at clubs on and off campus. They added more members, eventually growing to seven players, and went professional as a cover band called The Big Thing. The band featured an unusual and unusually versatile line-up of instrumentalists including saxophonist Walter Parazaider, trombonist James Pankow, and trumpet player Lee Loughnane along with more traditional rock instruments. While gaining some success as a cover band, the group worked on original songs and in 1968 moved to Los Angeles, California under the guidance of their friend and manager James William Guercio, and signed with Columbia Records. Upon release of their first record in early 1969, the band took a new name, Chicago Transit Authority (the name would almost immediately be changed to simply Chicago after the real CTA objected).

The band's first album, the eponymously titled Chicago Transit Authority, was an audacious debut: a sprawling double album (unheard of for a rookie band) that included jazzy instrumentals, extended jams featuring Latin percussion, and experimental, feedback-laden guitar abstraction. The album also included a number of pop-rock gems (several of which would later be released as singles and eventually become rock radio staples), and began to receive heavy airplay on the fledgling FM radio band.

The band's popularity exploded with the release of their second album, another double-LP set, which included several top-40 hits. This second album, unofficially titled Chicago II, was the group's breakthrough album. The centerpiece track was a 15-minute suite composed by James Pankow called "Ballet For A Girl In Buchannon" (the structure of this suite was inspired by Pankow's love for classical music). The suite yielded two top ten hits, "Make Me Smile", and "Colour My World". Among the other tracks on the album: keyboardist Robert Lamm's "25 Or 6 To 4" (sung by bass player Peter Cetera), and the lengthy "It Better End Soon". With that, the pattern had been set: the band, ever prolific, recorded and released music at a rate of more than two LP discs per year (always titled with the band name and a Roman numeral) from their debut in 1969 through the 1970s.

Some fans say a low point of the group's early career came when they released a quadruple-album live set, At Carnegie Hall, Vols. 1-4 (Chicago IV) (consisting of music from their first three albums). The performances and sound quality were judged sub-par; in fact, one group member went on record to say that "the horn section sounded like kazoos". The group bounced back from this misstep in 1972 with their first single-disc release, Chicago V, a diverse set that reached number one on both the Billboard pop and jazz albums charts and yielded the radio hit "Saturday In the Park".

Other successful albums and singles followed in each of the succeeding years. 1973's Chicago VI also topped the charts buoyed by hits "Feelin' Stronger Every Day" and "Just You and Me". Chicago VII, the band's double-disc 1974 release, featured the Cetera-composed "Wishing You Were Here" (sung by lead guitarist Terry Kath, with background vocals by The Beach Boys). The next year's release, Chicago VIII featured the political allegory "Harry Truman" and the nostalgic "Old Days". That summer saw a very successful joint tour across America with the Beach Boys, with each act performing some of the other's material. But for all their effort, none of their singles went to number one until the group's tenth album (Chicago X) in 1976, when Cetera's slow, exquisite ballad "If You Leave Me Now" went to the top of the charts. That was the song which won the group their only Grammy award for Best Rock Performance by a duo or group in 1977.