Casdagli was born in Greece. She discovered that she was a lesbian at her theatre school, which was a girls' boarding school. She left drama school in the late 1960s and hid her lesbianism. This was despite the fact that she worked at the Unicorn Children's Theatre where she had a powerful lesbian role-model in Caryl Jenner who ran the company. For a number of years she used the alias Maro Green.
In the mid-1970s she had her first experience of presenting a piece of lesbian writing to a heterosexual workshop funded by the Gulbenkian Foundation, and was discouraged by members of the workshop being quick to attack and dismiss her work. She found inspiration from the Women Live festival in 1980 when she witnessed the feminist-motivated movements within the theatre. She decided to come out as a lesbian to her colleagues when touring with an Alan Ayckbourn play. However, the response was unbearable with fellow actors harassing her and making life and work difficult. She resolved not to work in mainstream theatre again.
In 1985 her play More was presented as a rehearsal reading at the Gay Sweatshop Times Ten Festival, although she had decided to write under the name Maro Green. Everyone involved with the production of More was lesbian. This was the first of three collaborations with Caroline Griffin.
Because More was such a success Penny Casdagli and Caroline Griffin formed their own theatre company, Nitty Gritty to perform Memorial Gardens. The Arts Council refused to provide funding on the grounds that the director was inexperienced, but the company obtained funding by writing to well-known theatre people to ask for support. The final play by Penny Casdagli and Caroline Griffin was Mortal, performed by the Women's Theatre Group, but the director so misunderstood and misinterpreted the play that the result was a mangled mess. The outcome seemed to reveal what can happen when lesbians write a play but lose control of it and it is put in the hands of heterosexuals to put their own interpretation on it.
In 1987 Penny Casdagli received the British Drama Award for the Best Young People's Play for Parden Mr Punch. Also in 1987 Penny Casdagli and Caroline Griffin established the company "Neti-Neti" which was not overtly lesbian. The name Neti-Neti is a Zen term meaning 'not this and not that'. In Sanskrit it means 'neither one thing nor the other'. The name arose from the fact that the company was diverse with different ethnic groups, people with different sexualities, and people who were differently abled.
The company produced theatre for young people involving sign language, English, and Bengali. Because of this diversity the company was considered more favourably and received funding from the Arts Council. In 1991 British gas gave the company one of their small theatre awards. However, in 1992, even after appearing on thirteen television programmes, the Arts Council refused a project grant on the grounds on an unfavourable show report.
When it became apparent that none of the Arts Council panel had seen any of the company's shows for four years the company protested and received their grant. In 1995 the company produced Philip Osmen's play, Who's Breaking?, at the Lyric Theatre, Hammersmith. This was about coming to terms with AIDS, for men, women, homosexual, and heterosexual.
In 1995 Penny Casdagli left Neti-Neti for internal political reasons.
