Sarah Jane Macaulay was born in Beaconsfield, Buckinghamshire on 31 October 1963. Her mother Pauline was a Teacher and her Father Iain worked for publisher Longman. Macaulay spent her infancy in Fife, before her family moved to Tanzania—where her mother was to operate a school—when she was two years old. When she was eight, her parents separated. Both then remarried and her mother and stepfather took her and her two younger brothers, Sean and Bruce, to live in North London.

She was a founding partner of Hobsbawm Macaulay Communications, a public relations company. She is married to Gordon Brown, who served as Chancellor of the Exchequer from 1997 to 2007 and Prime Minister of the United Kingdom from 2007 to 2010.

Sarah's relationship with Gordon Brown started after sharing a FLIGHT from London to Scotland for the Scottish Labour Party conference in 1994. The relationship was kept secret until June 1997, when the News of the World published a picture of them together at a restaurant in London. They were married on 3 August 2000 in Brown's hometown of North Queensferry, Fife.

On 28 December 2001, she gave birth prematurely to a baby daughter, Jennifer Jane; who died at ten days old. Gordon spoke of Sarah's bravery after their daughter's death.

In 2002, Brown founded the charity Theirworld – originally known as PiggyBankKids – which began as a research fund to tackle complications in pregnancy, and in 2004 the charity founded the Jennifer Brown Research Laboratory at the University of Edinburgh. The laboratory's work is notable for its unified obstetric and neonatal approach to complications in pregnancy and childbirth, with a particular focus on preterm births.

On 17 October 2003, she gave birth to their second child and first son, John. Another son, James Fraser, arrived on 17 July 2006 and was diagnosed with cystic fibrosis that November.

Brown is also patron of the CBI First Women Awards, which since 2004 have celebrated "pioneering women; successful role Models who have broken new ground and opened up opportunities for other women".

Gordon Brown became Prime Minister on 27 June 2007, after the resignation of Tony Blair. As wife of the Prime Minister, Sarah Brown was generally viewed favourably. The Guardian noted her public image, describing her as "a truly modern public figure: talkative, empathetic, informal but infinitely connected, ubiquitous as any Celebrity, an avid exploiter of new digital media, an expert assembler of charitable and political coalitions", noting "her positive profile could be the best thing Labour has got going for it with the election looming." The Telegraph noted that " It is hard to find people with a bad word to say about Mrs Brown."

Sarah introduced Gordon at the 2008 and 2009 Labour Party Conferences. It was her idea to do so at the 2008 conference, after having seen the similar role Michelle Obama had performed for her husband, the United States President Barack Obama.

Brown chaired the launch of the "new consensus for maternal, newborn and child health" at a 2009 high level event at the United Nations. At the meeting 10 countries, including Sierra Leone, Ghana and Liberia, declared that they would be dropping medical charges ("user fees") to pregnant women around the time of birth. The consensus also set out key action steps that research showed could save the lives of more than 10 million women and children by 2015, and that were endorsed by the G8 at their July meeting of that year.

According to Anthony Seldon and Guy Lodge, the authors of a book on Gordon's tenure as Prime Minister, Sarah brought stability to both Gordon and his office, and was "a forceful voice in encouraging him to stay on until the very end". That end came following the 2010 General Election, which resulted in the first hung parliament since 1974. The Conservative Party led by David Cameron won the most seats, and on 11 May 2010 formed a government in coalition with the Liberal Democrats after her husband's attempts to keep Labour in power failed.

Brown published a memoir of her role as the Prime Minister's spouse, entitled Behind the Black Door, through Ebury Press in 2011.

The Irish Independent describes the book as a "disturbingly giddy, schoolgirlish, exclamation mark-littered diary form" and "nothing in the slightest bit revelatory about it... tiptoeingly discreet", but that "Brown comes across in these pages as a decent and likeable" with "enough gossipy details to satisfy star-hungry readers", with Caitlin Moran declaring: "School run, conference call, Obama for tea – Sarah Brown smiled, and tweeted, through it all. I love this woman."

Throughout her campaigning, Brown has been a leading user of social media to promote the causes of education and maternal health, and has been named on various Twitter and social media 'most influential' lists, including "The eight most influential women tweeters" by Forbes magazine, and in 2014, Brown was reported to be the second "most powerful Briton" on Twitter by The Independent.

In February 2015, it was announced that Sarah Brown would be competing in a second Comic Relief special edition of The Great British Bake Off television show, with The Guardian describing the line up as a "cause for celebration".