His time as Microsoft CEO has been seen as mixed, with critics noting the company's tripling of sales and doubling of profits, but losing its market dominance and missing out on 21st century Technology trends.

Ballmer was born in Detroit; he was the son of Beatrice Dworkin and Frederic Henry Ballmer (Fritz Hans Ballmer), a manager at the Ford Motor Company. His Father was a Swiss immigrant, and his mother was Belarusian Jewish. Through his mother, Ballmer is a second cousin of Actress and Comedian Gilda Radner. Ballmer grew up in the affluent community of Farmington Hills, Michigan. Ballmer also lived in Brussels for three years in 1964, where he attended the International School of Brussels. In 1973, he attended college prep and engineering classes at Lawrence Technological University. He graduated valedictorian from Detroit Country Day School, a private college preparatory school in Beverly Hills, Michigan, with a score of 800 on the mathematical section of the SAT and was a National Merit Scholar. He now sits on the school's board of Directors. In 1977, he graduated magna cum laude from Harvard University with a B.A. in applied mathematics and economics.

Steve Ballmer joined Microsoft on June 11, 1980, and became Microsoft's 30th employee, the first Business manager hired by Gates.

Ballmer was initially offered a salary of $50,000 as well as a percentage of ownership of the company. When Microsoft was incorporated in 1981, Ballmer owned 8% of the company. In 2003, Ballmer sold 39.3 million Microsoft shares equating to approximately $955 million, thereby reducing his ownership to 4%. The same year, he replaced Microsoft's employee stock options program.

One of his earliest known viral videos was his promotion of Windows 1.0 for a Crazy Eddie commercial in 1985, where he energetically shouts "How much do YOU think this advanced operating environment is worth? WAIT just one minute before you answer". Ballmer and Brian Valentine repeated this in a spoof promotion of Windows XP later on.

In 1990, he married Connie Snyder; they have three sons.

In the 20 years following his hire, Ballmer headed several Microsoft divisions, including operations, operating systems development, and sales and support. From February 1992 onwards, he was Executive Vice President, Sales and Support. Ballmer led Microsoft's development of the .NET Framework. Ballmer was then promoted to President of Microsoft, a title that he held from July 1998 to February 2001, making him the de facto number two in the company to the chairman and CEO, Bill Gates.

In July 2000, Ballmer called the free software Linux kernel "communism" and further claimed that it infringed with Microsoft's intellectual property. In June 2001 he called Linux a "cancer that attaches itself in an intellectual property sense to everything it touches". Ballmer used the notion of "viral" licensing terms to express his concern over the fact that the GNU General Public License (GPL) employed by such software requires that all derivative software be under the GPL or a compatible license. In April 2003 he even interrupted his skiing holiday in Switzerland to personally plea the mayor of Munich not to switch to GNU/Linux. But he did not succeed with this and Munich uses LiMux now, despite his offering a 35% discount at his lobbying visit.

Ballmer served as Director of Accenture Ltd. and a general partner of Accenture SCA from 2001 to 2006.

In 2005, Microsoft sued Google for hiring one of its previous vice Presidents, Kai-Fu Lee, claiming it was in violation of his one-year non-compete clause in his contract. Mark Lucovsky, who left for Google in 2004, alleged in a sworn statement to a Washington state court that Ballmer became enraged upon hearing that Lucovsky was about to leave Microsoft for Google, picked up his chair, and threw it across his office, and that, referring to former Google CEO Eric Schmidt (who previously worked for competitors Sun and Novell), Ballmer vowed to "kill Google." Lucovsky reports:

In 2007, Ballmer said "There's no chance that the [Apple] iPhone is going to get any significant market share. No chance."

On March 6, 2008, Seattle mayor Greg Nickels announced that a local ownership group involving Ballmer made a "game changing" commitment to invest $150 million in cash toward a proposed $300 million renovation of KeyArena and were ready to purchase the Seattle SuperSonics from the Professional Basketball Club LLC in order to keep the team in Seattle. However, this initiative failed, and the SuperSonics relocated to Oklahoma City, Oklahoma, where they now play as the Oklahoma City Thunder.

Speaking at a conference in NYC in 2009, Ballmer criticized Apple's pricing, saying, "Now I think the tide has turned back the other direction (against Apple). The economy is helpful. Paying an extra $500 for a computer in this environment—same piece of hardware—paying $500 more to get a logo on it? I think that's a more challenging proposition for the average person than it used to be."

During the 2011 Web 2.0 Summit in San Francisco, he said: "You don't need to be a computer scientist to use a Windows Phone and you do to use an Android phone ... It is hard for me to be excited about the Android phones."

As part of his plans to expand on hardware, on June 19, 2012, Ballmer revealed Microsoft's first ever computer device, a tablet called Microsoft Surface at an event held in Hollywood, Los Angeles. He followed this by announcing the company's purchase of Nokia's mobile phone division in September 2013, his last major acquisition for Microsoft as CEO.

In June 2012, Ballmer was an investor in Chris R. Hansen's proposal to build a new arena in the SoDo neighborhood of Seattle and bring the SuperSonics back to Seattle. On January 9, 2013, Ballmer and Hansen led a group of Investors in an attempt to purchase the Sacramento Kings from the Maloof family and relocate them to Seattle for an estimated $650 million. However, this attempt also fell through.

On November 13, 2014, it was announced that Ballmer would provide a gift, estimated at $60 million, to Harvard University’s computer science department. The gift would allow the department to hire new faculty, and hopefully increase the national stature of the program. Ballmer previously donated $10 million to the same department in 1994, in a joint-gift with Bill Gates.

In 2015, Ballmer called Microsoft's decision to invest in Apple to save it from bankruptcy in 1997 as the "craziest thing we ever did". By 2015 Apple was the world's most valuable company.

In March 2016, Ballmer changed his stance on Linux, saying that he supports his successor Satya Nadella's open source commitments. He maintained that his comments in 2001 were right at the time but that times have changed.

Ballmer launched USAFacts.org in 2017, a non-profit organization whose goal is to allow people to understand US government revenue, spending and societal impact. He is reported to have contributed $10 million to fund teams of researchers who populated the website's database with official data.