Lai was born on 26 April 1932, in Nice, France, the son of market gardeners of Italian origin.

After a short period with the orchestra of Michel Magne, Lai became an accompanist for Édith Piaf and composed for her.

[6] This initial success brought more opportunities to work for the film industry both in his native France, where he continued to work with Lelouch on scores to films such as Vivre pour vivre (1967), Un homme qui me plaît (1969), Le voyou (1970) and La bonne année (1973), as well as in Great Britain and the United States.

[1] In 1970 he wrote the score for director René Clément's film, Rider on the Rain ("Le passager de la pluie").

2 in the Billboard album charts and the film's theme, "Where Do I Begin", was a hit single with lyrics by Carl Sigman for singer Andy Williams.

This music is light to the point of almost not being there... Lai uses synthesizers, acoustic guitars, and a truckload of strings to weave what is supposed to be the "perfect" setting for two young lovers to do what young lovers do, but has it all coming out of the wash sounding like a commercial for those laundry detergents that are airy fresh, or a silent movie score where the music is supposed to tell you that this is a pensive moment.

"[17] His composition "Aujourd'hui C'est Toi" (Today It's You) is probably best known in the UK as the theme music for the long-running BBC current affairs documentary series Panorama.