In 1980, he along with his brothers Rakesh Mittal and Rajan Mittal started an Import Enterprise named Bharti Overseas Trading Company. He sold his bicycle parts and yarn factories and moved to Mumbai. In 1981, he purchased importing licences from exporting companies in Punjab. He then imported thousands of Suzuki Motors's portable electric-power generators from Japan. The importing of generators was suddenly banned by the then Indian Government.
In 1984, he started assembling push-button phones in India, which he earlier used to import from a Taiwan company, Kingtel, replacing the old fashioned, bulky rotary phones that were in use in the country then. Bharti Telecom Limited (BTL) was incorporated and entered into a technical tie up with Siemens AG of Germany for manufacture of electronic push button phones. By the early 1990s, Sunil was making fax machines, cordless phones and other Telecom gear. Sunil says, "In 1983, the government imposed a ban on the import of gensets. I was out of Business overnight. Everything I was doing came to a screeching halt. I was in trouble. The question then was: what should I do next? Then, opportunity came calling. While in Taiwan, I noticed the popularity of the push-button phone -- something which India hadn't seen then. We were still using those rotary dials with no speed dials or redials. I sensed my chance and embraced the Telecom Business. I started marketing telephones, answering/fax machines under the brand name Beetel and the company picked up really fast.". He named his first push-button phones as 'Mitbrau'.
In 1992, he successfully bid for one of the four mobile phone network licences auctioned in India. One of the conditions for the Delhi cellular license was that the bidder have some experience as a Telecom operator. So, Mittal clinched a deal with the French Telecom group Vivendi. He was one of the first Indian entrepreneurs to identify the mobile Telecom Business as a major growth area. His plans were finally approved by the Government in 1994 and he launched services in Delhi in 1995, when Bharti Cellular Limited (BCL) was formed to offer cellular services under the brand name AirTel. Within a few years Bharti became the first Telecom company to cross the 2-million mobile subscriber mark. Bharti also brought down the STD/ISD cellular rates in India under brand name 'Indiaone'.
In May 2008, it emerged that Sunil Bharti Mittal was exploring the possibility of buying the MTN Group, a South Africa-based telecommunications company with coverage in 21 countries in Africa and the Middle East. The Financial Times reported that Bharti was considering offering US$45 billion for a 100% stake in MTN, which would be the largest overseas acquisition ever by an Indian firm. However, both sides emphasize the tentative nature of the talks, while The Economist magazine noted, "If anything, Bharti would be marrying up," as MTN has more subscribers, higher revenues and broader geographic coverage. However, the talks fell apart as MTN group tried to reverse the negotiations by making Bharti almost a subsidiary of the new company. In May 2009, Bharti Airtel again confirmed that it was in talks with MTN and the companies agreed to discuss the potential transaction exclusively by 31 July 2009. Talks eventually ended without agreement, some sources stating that this was due to opposition from the South African government.
In September 2010, Mittal’s son, Shravin Mittal, joined Bharti Airtel as a manager having worked for Merrill Lynch in New York and Ernst & Young in London.
In 2012, Mittal took Bharti Infratel public with an initial IPO that raised $760 million. Mittal noted that the sale, considered by many to be only a modest success, was a “strong endorsement from qualified investors”. The board was restructured ahead of the IPO with Mittal remaining as Chairman and Managing Director. After the IPO, shares of Bharti Infratel dropped sharply at the commencement of trading.
Satya Bharti School Program' - the Foundation's flagship program is running 254 schools in six States serving over 43,000 rural children, free of cost. The other educational initiatives including the – Satya Bharti School, Quality Support and Learning Centre Programs, are currently reaching out to over 1,24,000 underprivileged children in 11 states. Two more Programs of the Foundation making considerable impact among the underprivileged sections are – 'Satya Bharti Abhiyan' (Sanitation) and 'Nyaya Bharti' (Legal aid).
In 2015, Sunil Mittal announced that he would be joining the board of Oneweb, a space internet company. Mittal was one of the Investors in a $500 million investment round that included Coca-Cola, Virgin and Qualcomm.
In 2016, Mittal made changes at Bharti Airtel to enable the company to compete against the launch of Reliance Jio Infocomm Ltd. in the race to become India’s largest telecoms company.
In 2017, Mittal jokingly announced that Bharti Airtel would be “declaring war on roaming” by scrapping charges for outgoing and incoming calls within India as well as international roaming charges.