As a multi-instrumentalist and songwriter with King Crimson, he helped propel the progressive rock movement. He found more commercial success with Foreigner.Ian McDonald, a multi-instrumentalist and songwriter whose work with the British band King Crimson helped propel the progressive rock movement of the 1960s and ’70s, and who went on to help found the immensely popular group Foreigner, died on Feb. 9 at his home in New York City. He was 75.The cause was colon cancer, said his son, Maxwell, who is also a musician.Though Mr. McDonald’s work with Foreigner reaped far greater financial rewards than his efforts with King Crimson — his trio of albums with Foreigner sold a combined 17 million copies — his earlier band far outdistanced them in creativity and influence. Their debut album, “In the Court of the Crimson King” (1969), with its radical sound and structure, was a watershed work in the history of rock. It was the only release in the band’s wide catalog in which Mr. McDonald had full involvement.For the album, he was a co-writer of every song, played nine instruments and provided primary production.“Ian brought musicality, an exceptional sense of the short and telling melodic line, and the ability to express that on a variety of instruments,” the band’s leader, Robert Fripp, wrote in the liner notes to a box set of King Crimson’s work released in 1997. In a recent email he said, “In 1969, I trusted Ian’s musical sense ahead of my own.”For the title track of “In the Court of the Crimson King,” Mr. McDonald wrote the music and provided a Mellotron hook that became one of the most recognizable uses of that instrument in rock history. Highly melodic, the sound provided a striking contrast to the fury of the album’s other most famous song, “21st Century Schizoid Man.” Fired by the combination of Mr. McDonald’s shrieking alto-saxophone and Mr. Fripp’s brutalist guitar, “21st Century Schizoid Man” both startled and thrilled listeners.Pete Townshend of the Who was so impressed, he wrote ad copy for the album in Rolling Stone magazine that read: “Twenty-first century schizoid man is everything multi-tracked a billion times, and when you listen you get a billion times the impact. Has to be the heaviest riff that has been middle-frequencied onto that black vinyl disc since Mahler’s 8th.”The album went gold in the United States and made the Top 5 in the band’s native Britain. At the end of a U.S. tour to promote the album, Mr. McDonald left the band, as did the drummer, Michael Giles. “I was probably not emotionally mature enough to handle it, and I just made a rather rash decision to leave without consulting anyone,” Mr. McDonald told Sid Smith in the 2001 book “In the Court of King Crimson.”In 1970, the two departed members released their own album, “McDonald and Giles,” which showcased Mr. McDonald’s vocal skills as well as his Beatle-esque sense of melody. That same year, several long songs he had helped write for King Crimson appeared on the group’s second album, “In the Wake of Poseidon.” In 1974, Mr. Fripp invited him back, and he played on two tracks of the band’s “Red” album, released that year. The band broke up shortly thereafter.King Crimson in 1969; from left, the leader and guitarist Robert Fripp, the drummer Michael Giles, the singer and guitarist Greg Lake, the multi-instrumentalist Ian McDonald and the lyricist Peter Sinfield. Michael Ochs Archives/Getty ImagesWhen work slowed for him in England in the mid-1970s, Mr. McDonald moved to New York, where he helped form Foreigner with another British transplant, Mick Jones.Foreigner’s debut album in 1977 made Billboard’s Top 5 list and sold more than five million copies. One song Mr. McDonald helped write, “Long, Long Way From Home,” became a Top 20 Billboard hit. He was a co-producer of all three albums that he recorded with Foreigner, including “Double Vision” (1978) and “Head Games” (1979).Ian Richard McDonald was born on June 25, 1946, in Osterley, Middlesex, England to Keith McDonald, an architect, and Ada (May) McDonald, a homemaker. His father played banjo and piano, and, in a house filled with music, Ian played guitar and piano.His multi-instrumental approach broadened at 15, when he left school and entered the British Army as a bandsman. “I was taught clarinet, and from there I taught myself flute and saxophone,” he told Big Bang Magazine in 1999. “I was exposed to a number of different musical styles.”After leaving the army and moving to London, he met Mr. Fripp, who had a whimsical group called Giles, Giles and Fripp, including Michael Giles and his brother Pete. Mr. McDonald recorded songs with them before they evolved into King Crimson, with the singer and bassist Greg Lake replacing Pete Giles.Almost immediately a buzz grew around them, leading to an invitation to perform at a Rolling Stones free concert in London in Hyde Park; the event was originally meant to introduce the Stones’ new guitarist, Mick Taylor, but it wound up doubling as a salute to the musician he had replaced, Brian Jones, who had died two days earlier. Audience estimates for the show range from 250,000 to 500,000. In 2013, The Guardian reported that “King Crimson nearly stole the show.”The trailer for a forthcoming documentary titled “In the Court of the Crimson King” shows Mr. McDonald apologizing to Mr. Fripp for leaving the band so soon after it began. His run with Foreigner also ended when he was fired by Mr. Jones, who, Mr. McDonald said, desired more control. (Mr. McDonald did play with the band for its 40th-anniversary tour in 2017.)In 2002, he formed a group with other ex-members of Crimson (without Mr. Fripp) called the 21st Century Schizoid Band, which toured and released several live albums. Mr. McDonald issued a solo album in 1999, “Driver’s Eyes,” and formed a new rock-oriented group, Honey West, which featured his son, Maxwell, on their album “Bad Old World” (2017).In addition to his son, Mr. McDonald is survived by his sister, Linda Rice.Throughout his life he remained proud of how King Crimson’s debut album had endured.“One thing I tried to do as the main producer was to have every moment be able to be listened to hundreds of times so that, hopefully, the album would stand the test of time,” he told the entertainment blog The Los Angeles Beat in 2019. “Here we are 50 years later, and people are still talking about it.” More