Jenne Casarotto, who has died aged 77 after a short illness, created in Casarotto Ramsay what Hollywood likes to call a boutique agency. But any business that deals both with the estate of Tennessee Williams and with the films of Steve McQueen clearly exercises a benign cultural reach beyond that of many more self-important addresses. Who else could be effortlessly skilled at doing the best for a playwright such as Caryl Churchill and for a movie director such as Stephen Frears?

Jenne was always dedicated to the interests of her clients with a self-effacement that was quiet, calm and authoritative. It was amazing how completely she could give you her attention, given the number of others queueing at the switchboard. All her clients – Oscar winners or Oscar dodgers – were worthy of her time, regardless of fortune. For them she was supportive and cheerful, always ready to smooth the bumps of show business.

In recent, rackety years, when both Covid and cowardice have depleted British film and television just as much as British theatre, an occasional “It’s hard out there” was as downcast as she ever allowed herself to be. As one of her most accomplished writers, Christopher Hampton, observed: “On a first night or at a film festival premiere her amused and unruffled presence was more effective than a Valium.”

Born in London, she was the daughter of Barbara (nee Robertson), a physiotherapist, and John Goodwin, a cardiologist. Jenne (Jennifer) was an early escapee from straitlaced private education, leaving Queen Anne’s school, Reading, at the age of 16.

After a secretarial job at an advertising agency in London, she travelled in the US, and found work as an advertising copywriter in Cincinnati, Ohio. She discovered her interest in the arts when she was commissioned by the American novelist Leon Uris to research his life for an autobiographical novel, QB VII.

At dinner one evening, she sat next to the literary agent Douglas Rae, and in 1970 he took her first to work at CMA London (now ICM) and then for Robert Stigwood’s musical empire. Thanks to Rae, she was able to follow her real vocation: representing writers and directors in film, television and publishing. It was not until 1989, when she was 43, that Jenne founded an agency of her own.

Initially looking after gifted film-makers such as Frears, Neil Jordan and David Leland, the Casarotto Company developed its distinctive character, first when it expanded to take on production designers and directors of photography, and then in 1992, when it acquired, after the death of the formidable play agent Peggy Ramsay, a unique client list from the world of theatre. Alan Ayckbourn, Howard Brenton and Edward Bond all arrived together.

With the help of Peggy’s genial deputy, Tom Erhardt, and his equally discerning colleague, Mel Kenyon, Casarotto Ramsay spread quickly out across all the performing arts. Starting with just three employees, today it has 70.

On the day Jenne died, I realised that in the 30 years we had worked together, she and I had never had a cross word. Her advice was bespoke and her tact was frictionless. Frears admits that he always told young film-makers to take their problems to her: “She’ll sort you out.”

Producers told me later that she was a fierce negotiator, but to us, there was no sign of it. She fought on our behalf, but kept the ugliness of the fight from us. Jenne knew the right thing to do without having to think about it. Sometimes, on a Monday morning, she would ring me to say “I know I don’t personally represent you in theatre, but over the weekend I couldn’t resist reading your play.”

Part of her steadiness came from the happiness of her 55-year marriage to Giorgio Casarotto, who looked after the business side of the agency, and her pride in her sons, Mark and Daniel. She invited clients to a 60th birthday party in Giorgio’s family restaurant behind the Arsenale in Venice, and we saw something of the network of wonderfully Italian family relationships that filled her life with the security that she transmitted to the rest of us. A generous feminist, she was especially keen to improve the lot of women in an industry slow to admit them.

As Hampton said: “Everyone – the film companies, the theatre producers, as well as her clients – trusted and respected her.” Jenne liked good films and she liked good plays.

Nothing pleased her more, and she did not really care if the author was a client or not. Her opinion was incorruptible. Her greatest pleasure lay in facilitating good work, and her legacy is the family of like-minded agents she leaves behind.

She is survived by her husband, sons, three grandchildren, and her brother, Martin.

• Jenne (Jennifer) Jane Blair Casarotto, film, television and literary agent, born 27 July 1946; died 29 February 2024