Armstrong was born into a family of Irish extraction who, after her birth, moved to Bromsgrove and later to Birmingham. In her late teens, she became a nun in the Society of the Holy Child Jesus, a teaching order, in which she lived from 1962 to 1969. Once she had advanced from postulant and novice to professed nun, she was sent to St Anne's College, Oxford, to study English. Armstrong left the order while still an undergraduate. After graduating with a congratulatory First, she embarked on a DPhil on the poet Tennyson, but was failed by her external examiner. This period was marked by ill-health — her life-long but, at that time, undiagnosed epilepsy discussed in her autobiography The Spiral Staircase — as well as the difficult readjustment to outside life.

In 1976, Armstrong became an English teacher at a girls' school in Dulwich, but her illness caused so many days off work, that she was finally asked to leave in 1982. During this year she had published Through the Narrow Gate, a well-received account of her convent agonies. Largely on the strength of this, in 1984, Armstrong was commissioned by the British Channel Four to write and present a TV documentary on the life of St. Paul. Her subsequent stay in Jerusalem has been described by her as a "breakthrough experience", which defied her prior assumptions. Armstrong describes in The Spiral Staircase how all her work since has, in a sense, flowed from that comparatively brief period in Jerusalem. In 1996, she published Jerusalem: One City, Three Faiths. A major influence on Armstrong's whole approach to the world's religions has been, as she implies in The Spiral Staircase, the work of the Canadian scholar Wilfred Cantwell Smith.

The increasing interest in and debate surrounding Islamic issues has made Armstrong a popular speaker, causing some observers to credit her with being influential in conveying a "more objective" view of Islam to a wide public in Europe and North America.

The Great Transformation: The Beginning of Our Religious Traditions, was published in March 2006, and a measure of her popularity came that same year when she achieved a British accolade of being invited to choose her eight favourite records for BBC Radio's Desert Island Discs programme.

In 2007, Armstrong was invited by the Islamic Religious Council of Singapore to deliver the "2007 MUIS Lecture".

Armstrong is a fellow of the Jesus Seminar. She has written numerous articles for The Guardian and other publications. She was a key advisor on Bill Moyers' popular PBS series on religion, has addressed members of the US Congress, and was one of three scholars to speak at the UN's first ever session on religion. Karen Armstrong Speaker Profile at The Lavin Agency, thelavinagency.com She is a vice-president of the British Epilepsy Association, otherwise known as Epilepsy Action.

Armstrong, who taught for a time at London's Leo Baeck rabbinic college, says she has been particularly inspired by the Jewish tradition's emphasis on practice as well as faith: "I say that religion isn't about believing things. It's about what you do. It's ethical alchemy. It's about behaving in a way that changes you, that gives you intimations of holiness and sacredness." She points out that religious fundamentalism is not just a response to but, paradoxically, a product of contemporary culture. "We need to create a new narrative, get out of the rat-run of hatred, chauvinism and defensiveness; and make the authentic voice of religion a power in the world that is conducive to peace."

In an interview, Armstrong sharply criticized Robert Spencer for his book entitled The Truth About Muhammad: Founder of the World's Most Intolerant Religion, arguing that it is "written in hatred," contains "basic and bad mistakes of fact," and that the author "deliberately manipulates the evidence". Spencer responded that "Actually, it was Karen Armstrong... who committed 'basic and bad mistakes of fact' and perhaps 'deliberately manipulate[d] the evidence' in her truth-free review of my book..."

Christianity and Islam

In an interview with Bill Moyers, Armstrong compared Christianity to Islam, stating that "Islam is a religion of success. Unlike Christianity, which has as its main image, in the west at least, a man dying in a devastating, disgraceful, helpless death...Mohammed was not an apparent failure. He was a dazzling success, politically as well as spiritually, and Islam went from strength to strength to strength." Armstrong has also argued that “Until the 20th century, Islam was a far more tolerant and peaceful faith than Christianity. The Qur’an strictly forbids any coercion in religion and regards all rightly guided religion as coming from God; and despite the western belief to the contrary, Muslims did not impose their faith by the sword.”

Muslim Brotherhood

Regarding the Muslim Brotherhood, Armstrong stated in 2005 that:

"[The Muslim Brotherhood] set up a wonderful welfare program before it was suppressed. Factories where Muslims could work, had time for prayers, had vacation time, insurance, [learned] labor laws, [provided] clinics, they taught people how to treat sewage, drainage, and it was always the religions response to try to help modernity to give to the ordinary people the benefits of modernity in an Islamic setting that made sense to them and made things more balanced."

2005 London Suicide bombing

Regarding the Suicide bombings in London on July 7, 2005, Armstrong stated:

I thought that this [the attacks] was virtually inevitable. This is a political matter. And Tony Blair had put us right on the front line by joining with former President Bush. And we were all expecting this in London. There was no great surprise."

The Pope

Regarding the Pope, Armstrong has stated that "The Pope is the world's last, great, absolute monarch. He not only controls doctrinal and spiritual affairs, but also the political, social and economic fortunes of his church. And because he's believed to be directly guided by God, his decisions have the ring of absolute truth, which is strangely out of kilter with the democratic tenor of today's world.

Muslims and the Media

In an interview, Armstrong stated that "Muslims should try to use the media; they have got to learn to lobby like the Jews, and they have got to have a Muslim lobby, if you like ....this is a jihad, an effort, a struggle, that is very important. If you want to change the media, then you have got to make people see that Islam is a force to be reckoned with politically and culturally." She has also stated that "the West has got to learn that it shares the planet with equals and not with inferiors."

Israeli-Palestinian conflict

Regarding the Israeli-Palestinian conflict, Armstrong has stated that charges of anti-Semitism in Europe play into the hands of the Zionist lobby in America because "this will discredit anything Europe says. They say Europe is anti- Semitic because for the first time Europe is becoming aware of the plight of the Palestinians. It is part of a campaign to discredit European input in any future peace process." She has also argued that "At the moment there is no hope; they, the Israelis, can do what they want because America will always support them. I wish Europe would play a better role, but Mr Blair is running after Mr Bush like a poodle." She also suggested that "The problem with Israel now is that it cannot believe that it is not 1939 any more; the Israeli people are emotionally stuck in the horrors of the Nazi era."

Armstrong was honoured by the New York Open Center in 2004 for her "profound understanding of religious traditions and their relation to the divine."

In 2008 Armstrong was one of three winners who were awarded $100,000 each by the TED Conference's TED Prize. Her TED Prize wish was to initiate an international Charter for Compassion, crafted by a council of leading thinkers in the Abrahamic traditions of Judaism, Christianity and Islam, to help restore the Golden Rule as central to religious practice and daily life throughout the world.

In May 2008 she was awarded the Freedom of Worship Award by the Roosevelt Institute, one of four medals presented each year to men and women whose achievements have demonstrated a commitment to the Four Freedoms proclaimed by President Franklin D. Roosevelt in 1941 as essential to democracy: freedom of speech and of worship, freedom from want and from fear. The institute stated that Armstrong had become "a significant voice, seeking mutual understanding in times of turbulence, confrontation and violence among religious groups." It cited "her personal dedication to the ideal that peace can be found in religious understanding, for her teachings on compassion, and her appreciation for the positive sources of spirituality."

Journal articles:

• "Women, Tourism, Politics" (1977)
• "The Holiness of Jerusalem: Asset or Burden?" (1998)
• "Ambiguity and Remembrance: Individual and Collective Memory in Finland" (2000)

Books:

• Through the Narrow Gate (1982)
• The First Christian: Saint Paul's Impact on Christianity (1983)
• Beginning the World (1983)
• Tongues of Fire: An Anthology of Religious and Poetic Experience (1985)
• The Gospel According to Woman: Christianity's Creation of the Sex War in the West (1986)
• Holy War: The Crusades and their Impact on Today's World (1988)
• A Biography of the Prophet (1991)
• The English Mystics of the Fourteenth Century' ' (1991)
• The End of Silence: Women and the Priesthood (1993)
• A History of God (1993)
• Jerusalem: One City, Three Faiths (1996)
• In the Beginning: A New Interpretation of Genesis (1996)
• Islam: A Short History (2000)
• The Battle for God: Fundamentalism in Judaism, Christianity and Islam (2000)
• Buddha (2001)
• Faith After September 11 (2002)
• The Spiral Staircase (2004)
• A Short History of Myth (2005)
• A Prophet For Our Time (2006)
• The Great Transformation: The Beginning of Our Religious Traditions (2006) ISBN 978-037-541317-9
• The Bible: A Biography (2007)
• The Case for God (2009)