Kumar began his career at Hewlett Packard as a product manager before joining McKinsey and Company in 1986 as one of the earliest Indian-Americans at the consultancy. In 1988 he co-founded McKinsey's offices in Silicon Valley with a partner, growing the office "from two people to approximately 35" by 1992. In 1993 as a partner he founded McKinsey's office in New Delhi, which along with the Mumbai office co-founded the consultancy in India. In India he pioneered the concepts of Knowledge Process Outsourcing and Business Process Outsourcing and became the protégé of then-managing Director (chief executive) Rajat Gupta. Kumar returned to the United States to found and lead McKinsey's Internet practice during the dot-com bubble. "Under [Kumar's] leadership, McKinsey's e-commerce practice grew to represent a full 25-30% of the firm's revenues," court documents revealed, though Kumar was blamed for the evaporation of this Business in the subsequent dot-com collapse. He and Gupta briefly created a program to allow the firm to accept stock in lieu of consulting fees. Kumar was also Chairman of the Knowledge Center and Chairman of the Asia Center. He was a Director and corporate officer of the firm. He lived and worked from multiple offices in New Delhi, New York, and Silicon Valley, traveling over thirty thousand miles a month.

Kumar co-founded the Indian School of Business with Gupta in 1997, today ranked among the top 15 Business schools in the world by The Financial Times. Court documents revealed he personally raised approximately 35% of all money donated to the school and kept a low profile as co-founder: "a large part of the intellectual vision of ISB is in fact Anil's, a point not many outside ISB's leadership will ever be aware of."

As of December 2009, Kumar was no longer at the consultancy. In January 2010 he pleaded guilty to insider trading charges and was "the government’s star witness" in March 2011 against Billionaire friend and Galleon Group founder Raj Rajaratnam (United States v. Rajaratnam). In the sprawling case his involvement was unusual; according to a Reuters blog, "He’s the only informant who could be considered even more successful than Raj was, at least professionally if not in terms of raw cash. Raj had money, more money than he really knew what to do with, but Kumar had much more societal acceptance and prestige." He settled with the SEC in May 2010 for $2.8 million, the amount after gains he received from Rajaratnam through a Swiss bank account in a domestic worker's name. Gupta, Rajaratnam, and Kumar were all close friends and had founded the $1.3-billion private equity firm New Silk Route together, though Rajaratnam and Kumar withdrew before the firm began operation.

In 2010 he pleaded guilty to insider trading in a dramatic "descent from the pinnacle of the Business world." He was the government's first cooperator and most important witness "in two of the most important securities fraud trials in history" against close friends and Business partners Raj Rajaratnam, the Billionaire founder of the Galleon Group family of hedge funds, and Rajat Gupta, the former head of McKinsey and Company and a board member of Goldman Sachs and Procter and Gamble. Rajaratnam and Gupta were both convicted in separate high-profile Criminal trials. He was sentenced in 2012 by Judge Denny Chin to two years of probation in exchange for testimony against former friends Rajanatram and Gupta. Chin stated that "greed wasn't the motive in [Kumar's] case" and that "this was aberrational conduct ... Mr. Kumar has led a law-abiding and productive life." Federal prosecutors called Kumar "one of the best and most important cooperating witnesses" they had ever worked with.

As of 2012, he was working with the Baylor College of Medicine and Max Health care to start a medical university, teaching hospital, and nursing school in India, and also with the Hero Group to start an 8000-person engineering college in India.

International media, Business, and Finance industry observers have analyzed extensively Kumar's actions in aiding Rajaratnam. Consensus remains divided on the precise motivators of money, respect, and relationship, with The New York Times asking, "Why would people who seem to have it all — wealth, prestige, powerful jobs and infinite access to others with the same — risk that, and more, to provide inside information to the Sri Lankan-born Billionaire?" Rajaratnam's annual payments were estimated at less than 5% of Kumar's annual income (and just 1-2% excluding a one-time bonus), further raising the question of motivation. Prosecutors wondered "why an incredibly bright, highly accomplished, professional consultant, and senior partner at arguably the world’s leading consulting firm, who contributed considerable time to start the Indian School of Business and to other charities, would betray his profession’s core values." One media source questioned Kumar's sentencing report. Judge Chin would ultimately rule that "greed wasn't the motive in [Kumar's] case."

In 2015, an investigation noted that Mr. Kumar had illegally collected funds from insider trading in offshore accounts in the name of his domestic worker, Manju Das. The investigation alleged that Ms. Das had no knowledge of these accounts, which were created with identity documents falsified by Mr. Kumar; and that Mr. Kumar had paid Ms. Das far below minimum wage for several years in violation of US law.

Kumar was represented by the late Robert Morvillo, who had previously led Billionaire Martha Stewart’s defense in her own insider trading case.