Peggy Jay: Long-serving Labour member of the London County Council and guardian of Hampstead and its heath

Peggy Jay was a politician, a matriarch and a feminist, a woman who, before the phrase was invented, strove all her life to "have it all". A mother of four, she was for 30 years a Labour member of the London County Council and for nearly four decades the wife of the Labour politician Douglas Jay. But above all, she was the "Queen of Hampstead", guardian of Hampstead and its heath. In her time she was known, too, as the "Boadicea of County Hall". Harold Wilson, when he was Prime Minister, described her with more modified rapture as "a useful woman".

Peggy Jay was a politician, a matriarch and a feminist, a woman who, before the phrase was invented, strove all her life to "have it all". A mother of four, she was for 30 years a Labour member of the London County Council and for nearly four decades the wife of the Labour politician Douglas Jay. But above all, she was the "Queen of Hampstead", guardian of Hampstead and its heath. In her time she was known, too, as the "Boadicea of County Hall". Harold Wilson, when he was Prime Minister, described her with more modified rapture as "a useful woman".

Jay was also dubbed proto-Swampy by her irreverent grandchildren – a reference to her fierce campaign to save Hampstead's trees, on one occasion stopping the chainsaws by appearing on the street, all guns blazing, in her dressing-gown. She was a renowned fighter, a force to be reckoned with, whether defending her beloved Hampstead Heath, or campaigning for social justice. Tall, handsome and blessed with formidable and patrician charm, she was at the centre of political affairs all her life – a life which, although outwardly privileged, was not without heartache and bitter disappointments.

She was born in 1913 in Manchester, "a sunless place", she wrote in her 1990 autobiography, Loves and Labours. When she was five her father, Maxwell Garnett, was appointed General Secretary of the League of Nations Union and the family moved to Hampstead. It was a magical place for Peggy from the start. She called it her "homestead, the backdrop to the vicissitudes of my life". She spent her childhood there, and during all the years that followed she moved only a few hundred yards from its centre – always within minutes of the heath. In 1983 she was made chairman of the Heath and Hampstead Society.

To be a Garnett was to be one of a progressive, distinguished and charismatic tribe. And so it has continued: Jay's son Peter is a former ambassador to Washington, her niece Virginia Bottomley was a Tory cabinet minister, her daughters Catherine and Helen were the blonde and beautiful "Jay twins" who symbolised the swinging Sixties.

As a young girl Peggy had enjoyed the giddy trappings of upper-middle-class life, with dances and tennis parties in her Garnett grandparents' magnificent Hampstead house. However, her parents, who lived in an only slightly less grand house, believed in education for girls: Peggy Garnett went to St Paul's School, then to Somerville College, Oxford (to read Economics; she left after two years with a diploma) – coached for the entrance examination by the academically brilliant Douglas Jay, "the boy next door", then a young Fellow at All Souls, Oxford. She fell in love with him, and remained in love for the rest of her life. Despite his airy assertion that he thought monogamy was a sin, they were married in 1933 in Hampstead Parish Church.

Peggy Jay had joined the Labour Party when she was at Oxford. (She defected to the SDP in the 1980s but rejoined Labour a year before her death). Now, a fully fledged married woman and filled with reforming zeal, she became an active member of the party. In 1938, unaware of the difficulties ahead of juggling a new baby, Peter, with a political life, she was elected a member of the London County Council – at 25, the youngest ever at County Hall. She said in later years that it had never occurred to her to do anything but voluntary work. From an office in Pentonville Road, she began visiting poor families, their homes, schools and hospitals, and was appalled by what she found. It was not possible for people to live like this, she said; something must be done.

For the next 30 years she campaigned for better deals for homeless families, for better state education, for the welfare of children in hospital, for patients in mental hospitals. She was involved in the setting up of One O'Clock Clubs for young mothers living in tower blocks, and with the opening up of London's canals to the public. She did it all while bringing up four children, sometimes to comic effect: one afternoon she came rushing in from County Hall to hear her daughter saying on the telephone that Mummy was not in, "she's out giving a talk on the importance of mothers being at home when their children get home from school".

During the 1940s and 1950s she represented Hackney, then Battersea – she received the highest personal Labour vote in the new Greater London Council – but in 1967 she lost her seat. It was a crushing blow, and there was more to come. She had been on the short list for a life peerage; she was removed, she was told, because it would have been nepotistic to honour the wife of a cabinet minister. Ironically, a few months later, Douglas Jay lost his job as President of the Board of Trade over the issue of the Common Market, and the marriage, which had been under strain for years, finally ended in divorce in 1972.

She never showed any bitterness about either public or personal rejection but launched into a new period of her life with characteristic vigour. She was, among other things, chairman, in 1979, of a committee of inquiry into training of staff working with the mentally handicapped, an appointment which led Barbara Castle to say, "If I hadn't expected a controversial report, I wouldn't have appointed Peggy Jay."

In her later years her work was centred on Hampstead. She campaigned to save Burgh House as a community and arts centre, she fought to keep McDonald's from invading the High Street, she kept a vigilant eye on any development that might encroach on the beauty of the heath and its surrounding streets. A children's playground, an art gallery in Burgh House and a flower-covered pergola on the edge of the heath were all named after her, and she was made life president of the Heath and Hampstead Society.

Her ever-growing brood – there are to date 17 grandchildren and 12 great-grandchildren – was a great joy, especially as some of the fifth generation had chosen to live in Hampstead, and huge extended family holidays on the Isle of Wight were a lifelong delight. Witty, autocratic and wise, she held court until the end and died, a happy woman, just before her 95th birthday, in her daughter's house, in the Hampstead she loved.

Ruth Gorb

Hurrah for Peggy Jay, we thought, writes Nicola Beauman, when we moved to Hampstead 25 years ago, leaving behind an Islington that was still ripping out wrought-iron railings and replacing them with yellow-brick walls. For Peggy was Hampstead's guardian, chivvying the Heath and Old Hampstead Society and very soon chivvying us.

I, who hate meetings and any mention of madam chairman, used willingly to spend evenings discussing planning applications, tree preservation orders and how to stop property developers eyeing up the heath; this, after all, was not so long since the building of the Royal Free Hospital, and the proposal to put a motorway on stilts across what is now the south-western corner of the heath. Peggy even encouraged us to go out with a hammer and pliers hidden under the blankets of the pram. We would take down estate-agents' boards and report back to base; very soon there was a total ban on boards in NW3.

Peggy used to come round and talk to me about Hampstead over the ritual gin-and-tonic, and to my husband about politics over supper. She was always so wonderfully warm, always interested in everything that was going on.

She was also, for me, a rather romantic figure: her father – handsome, clever, a leader of men – had been a hero of E.M. Forster's, and for myself as well the Garnetts represent a certain kind of English glamour; this has been handed down to Peggy's numerous descendants, of whom she was deeply proud.

Sitting next to Peggy Jay on a rickety deckchair on the sea wall in the Isle of Wight was like being plugged into the campaigning mains of the 20th century, writes Julia Cleverdon.

As her son Peter Jay pointed out at her funeral, unlike her father, Maxwell Garnett, whose life was blighted by launching ideas two decades too soon, Peggy's ability was to spot unerringly a cause just as its time had come, and then marshal the arguments, harry the bureaucrats and begin the heavy lifting. Stopping hospitals from separating sick children from their mothers, taking mentally handicapped children out of institutions, protecting Hampstead Heath from the developers – her list was seemingly infinite and her campaigning experience honed on public committees for more than 70 years.

Her wise advice to me when first taking on a public appointment was: "Decide the three windmills you are going to tilt at. Never speak about any other item on the agenda. Get your voice into the air time in the first 10 minutes of the meeting. Ask questions where you know the answer, but which you want put on the record."

Life on the sea wall with her was a formidable masterclass in multi-tasking. Penetrating questions about politics and literary biography, a running commentary on house-proud relations, "If I hear one more discussion about which day the dustbins are left out", interspersed with staccato instructions to children on the sand – "Luke, give that bucket back at once!", "Tor, I'll give you 10p if you brush your hair" – and yet always her passionate belief that what you had to say was the only thing she really wanted to hear. Old and young left her matriarchal court feeling stronger, wiser, better able to cope.

If the world is divided, as Charles Handy believes, into drains and radiators, Peggy radiated hope and optimism that the world could be changed – and quite soon – if you could just get the hang of how to do it, and her life's work was doing just that.

Margaret Christian Garnett, politician and activist: born London 28 January 1913; member (Labour) for Hackney, London County Council (from 1965 Greater London Council) 1938-49, for Battersea 1952-67; Chairman, Heath and Hampstead Society 1968-89; married 1933 Douglas Jay (created 1987 Baron Jay, died 1996; two sons, two daughters; marriage dissolved 1972); died London 21 January 2008.