Eileen Halloran, aged 50, the former head of physical education at the City of London girls' school, took action when she discovered that she was being paid more than pound;7,000 less than her male counterpart at the nearby boys' school.
She is now expecting a five-figure sum in compensation.
Miss Halloran was head of PE at the school - which charges fees of pound;22,500 a year - from 1973 until 1995.
An industrial tribunal heard that she also earned less than two PE teachers at the City of London school - which caters only for boys - but who were not heads of department.
Miss Halloran was one of about 70 teachers at the girls' school - mainly female - being paid on a lower scale by the City of London Corporation which runs both of the private schools.
The tribunal rejected arguments put forward by the Corporation that lower pay in the girls' school reflected lower fees, ruling that this was in itself discriminatory.
The tribunal concluded that Miss Halloran's post was "comparable" with that of the boys' school head of sports, Ron Bailey, and that she should be paid the same salary, even though he was responsible for more pupils and more facilities.
It is estimated that about 50 of her former colleagues may now be entitled to have their pay brought into line with teachers in the boys' school.
Miss Halloran, who was represented by the Association of Teachers and Lecturers, said she was "jolly pleased" with the outcome of the hearing.
But she added: "My one disappointment is that the Corporation chose to fight this, rather than support the legitimate rights of staff."
Peter Smith, the ATL's general secretary, said nine other private employers of both boys and girls schools had already been identified as likely to be affected by the ruling.
He said: "I personally find it very surprising that the Corporation of London, which should in my judgment be attempting to get rid of a reputation for being male-dominated, should have resisted this challenge for so long, instructing a QC to represent them and, above all, that they should have resisted this in the field of education.
"What message are they sending to teachers in these schools? What message are they sending to children and to the wider world?" A spokeswoman for the Corporation said it was disappointed with the ruling, but a more formal response would only be made once the council's solicitor had met with the governors of the school.