Francis Lai, left, with director Claude Lelouch, center, and actress Anouk Aimee, right, in 2006. (Francois Durand/Getty Images)

Francis Lai, who won an Academy Award for his heart-tugging musical score of the blockbuster 1970 movie “Love Story” and who composed the music for more than 100 films, including the 1966 hit French film “A Man and a Woman,” died Nov. 7 in Nice, France. He was 86.

His death was announced by the mayor of Nice, Christian Estrosi. The cause was not disclosed.

Mr. Lai (pronounced “Lay”) began his musical career as an accordion player and as an accompanist to the renowned French chanteuse Edith Piaf. He was primarily a songwriter before being introduced to filmmaker Claude Lelouch, who invited Mr. Lai to compose a score for “A Man and a Woman” — and for another 35 films on which they worked together.

At a time in the 1960s and early 1970s, when the Vietnam War, student unrest and rock music were upending global culture and film, Mr. Lai composed movie soundtracks that were unabashedly romantic and melodic.

“His music was not just beguiling, but it conveyed a charming seductiveness,” film music historian Jon Burlingame said in an interview. “There was something sexy about it.”

The slender plot of “A Man and a Woman” revolved around two widowed parents — a film assistant played by Anouk Aimee and a racecar driver played by Jean-Louis Trintignant — who fall in love after being brought together by their children, who attend the same school. The film, shot in black and white, includes scenes of racing, walks on the beach, all-night drives in the rain and breathless reunions of the two central characters.

Mr. Lai in 2017. (GABRIEL BOUYS/AFP/Getty Images)

A Man and a Woman” received decidedly mixed reviews — critic Judith Crist called it a “simple-minded cinematic charlotte russe” — but audiences couldn’t get enough of the film’s romantic yearning or its captivatingly beautiful co-stars. The film shared the grand prize at the 1966 Cannes Film Festival.

Another key element was Mr. Lai’s mesmerizing music, which was an essential part of the film’s underlying emotional pulse. The principal theme is built on three rising notes, followed by two rapid passages of five notes each — lah-dah-dah, ba-da-ba-da-ba, lah-dah-dah, ba-da-ba-da-ba — repeated in countless variations. Mr. Lai’s accordion, supported by low-key vocals, piano and orchestration, made the music indelible, creating an aural rhythm that paralleled the scenes on the screen. The soundtrack album became an international best seller.

“It’s inimitably French,” Burlingame said. “It’s the imagery and the music together that make a film memorable.”

Mr. Lai’s musical style owed a great deal to traditional French music, jazz and bossa nova, but he also sometimes wrote in an imitation-classical style or experimented with synthesizers and other electronic instruments. He always composed at night.

He wrote the soundtracks for Terence Young’s “Mayerling” (1968), René Clément’s “Rider on the Rain” (1970) and the 1978 equestrian movie “International Velvet,” with Anthony Hopkins and Tatum O’Neal. His other credits included music for the soft-core erotica movies “Emmanuelle II” (1975) and “Bilitis” (1977); in all, Mr. Lai’s soundtrack albums sold more than 20 million copies.

Because Mr. Lai did not speak English and seldom left France, he had limited exposure in Hollywood before “Love Story,” a 1970 film directed by Arthur Hiller and based on a blockbuster novel by Erich Segal.

Mr. Lai twice turned down offers to work on “Love Story.” He was finally persuaded by French actor Alain Delon, who flew to Paris with Paramount studio head Robert Evans to show Mr. Lai a rough cut of the movie.

“I came out of the screening incredibly moved,” Mr. Lai said in a statement to the Los Angeles Times in 2001. “I went straight home, sat at my keyboard and wrote that theme that very night.”

The haunting score, much of it played on piano, deepens the heart-tugging quality of the movie, which featured Ali MacGraw and Ryan O’Neal as college lovers. “Love Story” was nominated for seven Oscars, but the only one it won was for Mr. Lai’s score.

The soundtrack album reached No. 2 on the Billboard chart. Andy Williams’s vocal version of the theme song, “Where Do I Begin?” — recorded after the film was made, with lyrics by Carl Sigman — was a Top 10 hit.

“The tune itself has an inimitable sadness to it,” Burlingame said. “There’s something romantic and melancholy, and if you combine them, deeply emotional — which makes him perfect for film.”

Francis Albert Lai was born April 26, 1932, in Nice. His parents were gardeners who sold produce. A cousin taught him to play the accordion, and he later learned the piano.

After moving to Marseille, France, he became an accompanist to singer Claude Goaty and became a devotee of jazz. He later followed Goaty to Paris, then became part of Piaf’s circle. He worked as her accompanist and wrote about 600 songs, some of which were performed by Piaf, Juliette Greco and Yves Montand.

Survivors include his wife of 50 years, Dagmar Puetz; and three children.

Through the years, Mr. Lai developed an unusual working arrangement with Lelouch on their many films, which include “Live for Life” (1967), “And Now My Love” (1974) and “A Man and a Woman: 20 Years Later” (1986).

In most cases, Lelouch outlined the plot for Mr. Lai, who then wrote the musical themes before a single scene had been shot. The music helped shape the direction and mood of the films.

“In every one of these settings, Lai found a new musical signature that was right for each film,” Burlingame said. “He seemed to be an endless fountain of melodies.”

His final project with Lelouch, “Les plus belles annees,” which reunited Aimee and Trintignant more than 50 years after “A Man and a Woman,” is scheduled for release in 2019.

“He was an angel disguised as an accordionist,” Lelouch told a French radio network after Mr. Lai’s death. “He made the heart beat in all my films.”