The trade union leader Mary Turner, who has died from cancer aged 79, had a particular set of personal skills that enabled her to survive a lifetime in the difficult and challenging world of trade union politics while engendering respect and affection on all sides. As president of the GMB for 20 years, where she was known as “our Mary”, she spoke truth to the people she represented, as well as to power.

When she was elected in 1983 to the central executive council of the General and Municipal Workers’ Union (now the GMB), Turner was the only woman among 40 members. More than two decades later, and as president since 1997, she saw the number of women on the executive increase to 26 out of 55.

She was the authentic voice of the ordinary man and woman, the hospital porter, the office cleaner, the shop assistant and the dinner lady. She spoke a commonsense language they understood, and they knew that she knew what she was talking about, because she had been there and done those sort of jobs herself.

A former school dinner lady at Salusbury primary school in Brent, north London, and a fervent socialist, Turner campaigned for 40 years for free hot meals at lunchtime for schoolchildren. The introduction of free meals for infants, implemented as part of the Liberal Democrats’ policy programme for the coalition with the Conservatives in 2010, was a consequence of Turner’s unremitting pursuit of this cause, but to her fury and dismay was destined for abolition in the recent Tory manifesto. The proposal was abandoned, however, as a result of Theresa May’s failure to secure an overall majority, an outcome in which Turner rejoiced.

Turner was an active member of the Labour party throughout her adult life and was elected to represent her union on the party’s national executive in 1995. She chaired the NEC from 2003 to 2004. When the last Labour government introduced a policy initiative called The Big Conversation during that time, it included the appointment of more women on the boards of the country’s top companies among its lists of objectives. Turner wanted to know why it did not, rather, say more about equal pay.

She was not an enthusiast for Tony Blair’s New Labour project and did not hesitate to make her views known on the private finance initiative, which she was vehemently against. She also badgered Blair to take the initiative on free school meals.

As chair of the Labour party’s policy forum, she worked with Ed Balls as shadow chancellor to secure the inclusion of free school meals in official party policy. But she ticked him off in 2012 for refusing to commit Labour to reverse George Osborne’s approach to public-sector pay restraint, and she told Ed Miliband in 2013 that his planned union reforms would have Keir Hardie turning in his grave. She formed a warm friendship with Jeremy Corbyn, shortly after he was elected as an MP in 1983, and they campaigned together to resist a move to cut back on school meals provision in Barnet.

She was a familiar figure at Westminster, while socialising with her many friends in the Labour movement. One such recalled her prestigious capacity for drinking into the small hours and “being fresh as paint the following morning”.

Turner made two attempts to stand for the House of Commons herself. She opposed Ken Livingstone when he faced reselection in his Brent East constituency in 1989, in a contest he won with a two-thirds majority. A decade later she was one of six on the shortlist to replace him after he became mayor of London, and was runner-up in a hard-fought contest for the selection, which was won by the Brent council leader, Paul Daisley.

This was probably to the benefit of the GMB. It was facing a possible merger with the Unite trade union, and Turner’s relaxed style of leadership was credited with helping the union through a lively period of political in-fighting.

As chair of the union’s conference over many years, she was loved for her humorous style and lack of adherence to procedure. “Get a grip, Mary!” she would admonish herself if she made a mistake. She was still a vocal presence this year, despite her evident poor health.

Facebook Twitter Pinterest Mary Turner showing off a temporary tattoo to mark the tattoo and piercing industry’s union joining the GMB. Photograph: Fiona Hanson/PA

Her instincts owed much to the experience of her parents, as well as her own life story: her mother worked in childcare and her father was a cleaner and a security guard. Born Mary O’Brien in Thurles, Co Tipperary, in Ireland, she was educated initially at a convent school. Her parents moved to the north of England in the early 1950s and later settled in Kilburn, north London, where Mary went to Carlton Vale secondary modern school.

She left at 16 to train as a bookkeeper at Jackson’s Tailors in Oxford Street, joining the National Union of Tailors and Garment Workers, now part of the GMB. She also worked in the print industry, joining Sogat and becoming mother of the chapel of her branch.

She married Denny Turner, a painter and decorator, in 1956, and took time out from work while their children, John and Denise, were small. She returned to work as a dinner lady in 1970 and immediately set about recruiting her colleagues into membership of the General and Municipal Workers’ Union, persuading them that it was worth the ninepence weekly subscription. One of her early successful campaigns was to secure the routine issue of rubber gloves to kitchen catering staff.

Thereafter she became increasingly active in the London region of the GMB. She was secretary of the Hendon branch for 26 years and a member of the union’s regional council. As a shop steward she was proud of her ability to mobilise a small army of secondary pickets to support industrial disputes or confront political opponents such as the National Front. During the 1981 People’s March for Jobs, she organised a mobile field kitchen to feed the unemployed marchers.

She was good at rostrum rhetoric as well as the small stuff, skills that were rewarded when she was elected to the union’s executive in 1983. She fought for those who needed help, for part-time workers, for the low-paid and for protection against unfair dismissal. Latterly she campaigned for those suffering from dementia, the cause of her husband’s death in 2015.

She was appointed MBE in 2010 and CBE in 2016. She was an annual speaker at the Trades Union Congress and was awarded the TUC women’s gold badge in 2012 and the GMB’s Eleanor Marx award in 2016.

She is survived by John and Denise, and five grandchildren.

• Mary Josephine Turner, trade unionist, born 15 June 1938; died 19 July 2017