Garner is married to Carol and they live on the Lower North Shore in Sydney in the suburb of Roseville. He was ordained as a Methodist minister in 1982 and was a minister at Plymouth, Darlington, Llandudno and Bolton. Garner completed his Master of Theology at Oxford University in 1995, and has preached in Africa, Iceland, Europe, North America and Asia. Garner has been a frequent speaker, whilst in the UK, at festivals and conventions including Cliff College. Garner has been a Rotarian since 1995 and is a member of Sydney Rotary Club and the Australian Club.
In August 2015, as Garner's appointment as Superintendent/CEO of Wesley Mission was extended by the Wesley Mission Council until at least 2020, he said he saw this as the "third phase" of his tenure. The first five years marked his arrival and accumulative understanding of Wesley Mission's rich heritage, present complexity and potential for future development. The following five years he interpreted as building bridges: strengthening relationships and earning the trust of service partners; the third and current phase he wants defined by growth. "Our Word and deed integration is critical in moving forward," he said in a message to congregations.
Through his role at Wesley Mission, Garner is closely involved with welfare and service provision to the aged, families, the unemployed and homeless, those experiencing financial stress, addictions and personal crises. His contribution was recognised with his inclusion in the Government's Australia 2020 Summit in 2008.
In August 2015, Garner launched Wesley Mission’s new program specially designed to assist Indigenous communities in preventing suicide, explaining that the method of delivery was important. “Instead of a trainer facilitator standing in front of a room saying, ‘this is what you should do’, it's really much more of a ‘yarning circle’ where people together sit down and share their experiences and in a much longer way are allowed to tell those stories, feel the pain and know what it is to share together and share the experiences of their community," he said. The launch of the Aboriginal and Torres Strait Islander Wesley LifeForce Suicide Prevention Program coincided with the annual Wesley LifeForce Suicide Prevention Networks conference which drew more than 100 representatives from 60 local networks from across Australia. Among the Wesley LifeForce networks is the Gunyangara community which in 2007 had the highest pro rata suicide rate in the world. For the past two years not a single death by suicide has been recorded in that community. This local network is one of many across Australia where Wesley Mission has been able to partner local groups to find local strategies to prevent suicide. Garner told the annual Wesley LifeForce Memorial Service near the Sydney Opera House, attended each year by hundreds of people, that suicide “affects people from all walks of life, of all ages, and it affects many, many people – more than 2000 people in Australia bring their lives to an end each year”.,
In 2014, Garner and NSW Premier Mike Baird signed the first contracts in “the most significant reform of homelessness services in a generation”. Garner said the Going Home Staying Home program – for which the NSW Government is spending more than half a billion dollars over three years with partners such as Wesley Mission – was an opportunity to break the cycle of inter-generational homelessness and that he was pleased to see more funding directed to suburban and regional areas given Wesley Mission’s research that more than 70 per cent of homeless people using inner-Sydney services come from suburban and regional areas. “We are now moving from ad hoc and piecemeal approaches to homelessness to a more strategic and cohesive model,” Garner said. 
Launching the Wesley Report, Giving Disability Carers a Break, in May 2014, Garner said Wesley Mission's research found that more than a third of families in Australia had a person with a disability and that the role of carer "was hugely taxing and takes its toll on even the most robust, loving people."
Interviewed in the media about Wesley Mission's early intervention program in south-west Sydney for young mothers with a mental illness, Garner said the intervention program sought to help mothers to develop resilience "and remain connected with networks of support and friendship". He was happy the NSW Government had made mental health a policy priority.
In August 2015 the whole of the Wesley Mission family celebrated the 200th anniversary of the arrival of Rev Samuel Leigh, Australia’s first Methodist minister, with a celebration service at the Wesley Centre in Sydney. To mark the occasion Garner published a biography, Samuel Leigh: The first Methodist missionary to Australia and New Zealand. “The legacy Samuel Leigh has left is not just about the churches and ministries he instigated or set up; it is as much about his sacrificial commitment to the work of God, his determination to continue in his ministry despite heartbreaking obstacles, and his humble and enduring faith,” Garner said.
The documentary John Wesley: The Man and his Mission, co-written and presented by Garner, won Third Place for Best Documentary at the International Christian Film Festival (ICFF) in 2013. Garner journeyed throughout England to trace the story of the founder of Methodism in this “beautifully produced documentary” released by UK-based distributor Gateway Films/Vision Video. A review describes the documentary, directed by Richard Attieh, as “an inspiration to Christians and principled humanitarians of all faiths, highly recommended especially for public library and church DVD collections”.
In 2013 Garner was interviewed on the ABC about the key Wesley Mission report, Homelessness and the next generation, which reported a change in the demographic of those experiencing homelessness. He expressed concern that action on homelessness would be overlooked by both sides of politics in the bid to win government. In May 2012 Keith Garner spoke about the Wesley Mission report, Keeping minds well: Caring till it hurts. He has written A Word for the World (2013) and Cross Talk – Words for the Crossroads of Life (2008). He regularly hosts television programs produced by Wesley Mission which are shown on Channel 9 and the Australian Christian Channel. He was involved in the film Santa’s Forgotten People, a documentary about the Wesley Mission's work with the homeless at Christmas. Garner is a recognised spokesperson for welfare issues and frequently speaks on ABC Radio with John Cleary, discussing issues such as financial stress and homelessness in Australia. In late June 2013, marking 50 years since the inception of the Lifeline Suicide Prevention service, Keith and Carol Garner welcomed ABC radio presenter, Simon Marnie, to their home for Sunday Brunch, part of the Weekends with Simon Marnie program on ABC 702. He also writes opinion pieces in the mainstream media The Sydney Morning Herald. One article from 2012, supporting the community drive to protect public holidays, was widely quoted by work/life balance advocacy groups, such as the Facebook community group, Take the Time. Another article, marking the 2013 birth of Prince George of Cambridge, contrasted the life of a royal baby with that of a child born to a family struggling as part of Australia's "working poor".
Wesley Mission's most noted evangelical leader, Rev Dr Sir Alan Walker's life and work was featured in Garner's book, The Authentic Australian Evangelist: Alan Walker, published in March, 2014. The release of the book was accompanied by an address at Wesley Church on 13 March 2014. In his address, Garner said Walker, called "the conscience of the nation" by the former Governor-General, Bill Hayden, "will always be remembered for his strong Christian leadership … and a preparedness to stand beside the most vulnerable and disadvantaged no matter the personal cost. He also had a willingness to take a stand on what were then unpopular causes, but which later became orthodox thinking.” The address is available on DVD.
Garner writes a regular blog and hosts a weekly Christian broadcast on Wesley Impact TV via YouTube. His 2015 Easter Address was explored in depth on the ABC's Religion and Ethics Report.