Moritz was born to a Jewish family in Cardiff, Wales. He was educated at Howardian High School in Cardiff before moving on to Christ Church, Oxford, where he earned a Bachelor of Arts in history. In 1978, he received a Master of Business Administration degree from the Wharton School of the University of Pennsylvania as a Thouron Scholar.

Moritz first worked for many years as a Journalist. When he was a reporter for Time magazine, Steve Jobs contracted him in the early 1980s to document the development of the Mac for a book he was writing about Apple. According to Andy Hertzfeld, Jobs stated that "Mike's going to be our Historian," a comment made in response to the fact that a year earlier a history had been written about another computer company. As he was close in age to many on the development team, he seemed to be a good choice. By late 1982, Moritz was Time Magazine's San Francisco Bureau Chief and working on the special Time Person of the Year issue. His work on that issue (which was initially supposed to be about Jobs) included a lengthy interview with Jobs' high school girlfriend, Chrisann Brennan, in which she discussed the history of their child, Lisa. Moritz's follow up interview with Jobs on the subject led to denial of paternity on his part. The issue also contained negative commentary on Jobs from other Apple employees. The special issue was later renamed Machine of the Year prior to publication, celebratedThe Computer and declared that, "it would have been possible to single out as Man of the Year one of the Engineers or entrepreneurs who masterminded this technological revolution, but no one person has clearly dominated those turbulent events. More important, such a selection would obscure the main point. TIME's Man of the Year for 1982, the greatest influence for good or evil, is not a man at all. It is a machine: the computer." Jobs cut off all ties with Moritz after the issue was published and threatened to fire anyone who communicated with him. According to Hertzfeld, "some of us talked with Mike again surreptitiously, as he was putting the finishing touches on his book around the time of the Mac introduction" and the resulting text, The Little Kingdom: the Private Story of Apple Computer, "remains one of the best books about Apple Computer ever written."

In 1986, he joined Sequoia Capital after co-authoring "Going for Broke: The Chrysler Story" (with Barrett Seaman, TIME's Detroit bureau chief). After leaving Time, Moritz co-founded Technologic Partners, a Technology newsletter and conference company.

His internet company Investments include Google, Yahoo!, Skyscanner, PayPal, Webvan, YouTube, eToys, and Zappos. He currently sits on the boards of; 24/7 Customer, Earth Networks, Gamefly, HealthCentral, Green Dot Corporation, Klarna, Kayak.com, LinkedIn, Stripe and Sugar Inc.. Moritz previously served on the boards of A123 Systems, Aricent Group, Atom Entertainment, CenterRun, eGroups, Flextronics, Google, ITA Software, Luxim, PayPal, Plaxo, Pure Digital, Saba Software, Yahoo!, and Zappos. Google was one of several co-investments with John Doerr of rival venture capital firm Kleiner Perkins Caufield & Byers, and the initial public offering of the company in 2004 made him one of Wales' richest men. His investment in Google helped him achieve the number one listing in Forbes' "Midas List" of the top dealmakers in the Technology industry in 2006 and 2007, and a place on the 2007 "TIME 100". He ranked number 2 on the Midas List for 2008 and 2009. He is listed by The Sunday Times as having a fortune of UK£558 million.

On 18 June 2008, Michael Moritz and his wife, American Novelist Harriet Heyman, announced a donation of US$50m to Christ Church, Oxford, his former college, the largest single donation in the college's history.

In 2009, 25 years after "The Little Kingdom," Moritz published a revised and expanded follow-up: Return to the Little Kingdom: How Apple and Steve Jobs Changed the World. In the prologue to Return to the Little Kingdom, Moritz states that he was as incensed as Jobs was about the Time Magazine special issue: "Steve rightly took umbrage over his portrayal and what he saw as a grotesque betrayal of confidences, while I was equally distraught by the way in which material I had arduously gathered for a book about Apple was siphoned, filtered, and poisoned with a gossipy benzene by an Editor in New York whose regular task was to chronicle the wayward world of rock-and-roll music. Steve made no secret of his anger and left a torrent of messages on the answering machine I kept in my converted earthquake cottage at the foot of San Francisco’s Potrero Hill. He, understandably, banished me from Apple and forbade anyone in his orbit to talk to me. The experience made me decide that I would never again work anywhere I could not exert a large amount of control over my own destiny or where I would be paid by the word. I finished my leave [and] published my book, The Little Kingdom: The Private Story of Apple Computer, which I felt, unlike the unfortunate magazine article, presented a balanced portrait of the young Steve Jobs."

In July 2010, Moritz was conferred an Honorary Fellowship from Cardiff University, where his Father Alfred had previously been Vice-Principal and Professor of Classics.

On 11 July 2012, it was announced Moritz had donated £75m to Oxford University to support students from families with an income below £16,000 per year.

Moritz was appointed Knight Commander of the Order of the British Empire (KBE) in the 2013 Birthday Honours for services to promoting British economic interests and philanthropic work.

In November 2014, Moritz was given the Honorary Doctorates (Doctor of Letters honoris causa) from the HKUST based on his outstanding in recognition of his distinguished achievements and contributions.

On 16 October 2016, The Guardian reported Michael Moritz "donated $49,999 to a divisive ballot measure intended to clear San Francisco’s streets of homeless encampments, according to campaign filings."