In 1995, he invested in the first Internet Service Provider in France, World-NET, created the previous year by Sébastien Socchard, Philippe Langlois and Pierre Séguret. This company was sold the year before the internet bubble burst.

After taking over a company subsequently renamed Iliad SA, Xavier Niel launched in 1996 the 1st reversed directory service on the Minitel. This service was responsible for 1 million euros of revenue per year for Iliad until the end of the minitel in France in 2012. Niel remains Iliad’s majority shareholder, holding 71% of the share capital.

In 1999, he created Free, a French Internet service provider. The name came from the fact that modem access was really free, which caused some stir among other (non-free) French Internet providers, as well as it raised some questions about Free’s business model. In 2002, Free launched a broadband package at a low price (€29.99 per month), which became the benchmark in the market. At the same time, Free developed and launched the Freebox: the first triple-play multi-service box in France. The free modem service had brought to the company a strong potential customers portfolio, many of them switching to the broadband for the sake of comfort.

In 1999, he founded Online, a French hosting company.

On 28 May 2004, Xavier Niel was indicted and detained for a month for alleged procuring and misuse of company assets, which occurred in several sex shops in which he was a shareholder. On 30 August 2005, a Nolle prosequi was pronounced for the procuring part of his indictment, while on 13 October 2006, he was given a 2 years suspended prison sentence for the misuse of company assets.

In March 2010, Xavier Niel cofounded with Jeremie Berrebi Kima Ventures, a fund dedicated to invest in 50 to 100 startups a year everywhere in the world. Kima Ventures already invested in 330 companies from February 2010 to August 2014 in 32 countries. Business Insider described Xavier Niel and Jeremie Berrebi as almost certainly the most active angel investors in the world.

In late 2010 Niel was a member of a group of investors which acquired a controlling stake in the newspaper Le Monde.

In 2012, he created Free Mobile, which offered unlimited voice calls, text and data.

In 2013, Niel created a school named 42, which is a free tech school with no teachers, no books, no tuition for 1000 people every year.

In 2014, Cable & Wireless Communications (CWC) sold its 55% economic interest in Monaco Telecom to NJJ Capital, an investment vehicle controlled by Xavier Niel for US$445 million.

On December 18, 2014, via its holding company NJJ Capital, he acquired for 2.7 billion dollars (2.8 billion Swiss francs) Orange Switzerland (owned by Apax Partners since 2012), which was renamed Salt Mobile SA in April 2015. In May 2019 Salt Mobile SA sold 90% of its towers for 800 million dollars to Cellnex

In August 2015, Wired named him as the seventh most influential personality in technology in the world.

In February 2017, Vanity Fair magazine named him the most influential French in the world abroad.

In September 2018, he bought 100% of the Cypriot operator MTN Cyprus through Monaco Telecom for 260 million euros, the carrier was rebranded into Epic in June 2019.

In April 2018, through NJJ he personally invested 330 million euros for 32.9% of the capital of Eir. Iliad, for its part, took a 31.6% stake in Eir’s capital with a call option to buy back 80% of NJJ’s stake in 2024.

In April 2018, via NJJ, he acquired a stake in the capital of Tigo Senegal through the “Teyliom” consortium, a company which will was renamed Free Senegal in October 2019.

In 2018, the Iliad SA share price collapsed, going from € 212 in January 2018 to € 95 in November 2018. After restructuring its assets leading to the creation of the holding HoldCo, Xavier Niel did major maneuvers to reconquer the markets and strengthen its position in the capital of Iliad . In November 2019 he bought back at € 120 more than 11.7 million Iliad shares (i.e. 20% of the company’s shares) via a public share buyback operation, automatically raising the price to this level . Thanks to this operation financed by a loan guaranteed on Iliad shares held in his holding company Holdco, he increased his control from around 52% to 72%.

In March 2020, he bought 100% of the operator Vodafone Malta through Monaco Telecom for 250 million euros, market leader for telecommunications operators in Malta, in the fields of mobile, internet and fixed communications.

In September 2020, through Iliad, he launched a public offer for the Polish operator Play, leader of the Polish mobile market with 15 million subscribers after having bought a 40% stake from the two main shareholders. If all the shareholders sell their shares, the French group will have to spend 2.2 billion euros. Including the debt, the operation rises to 3.5 billion euros.

In October 2020, through his holding NJJ he buys shares on the stock market of Unibail-Rodamco-Westfield with Leon Bressler and Susana Gallardo, the consortium announced in late October 2020 that their stake in the company exceeds 5% , following their announcement the consortium was able to cancel the company’s plan of capital increase following a shareholders vote in early November 2020, consequently they gained three seats at the company’s board. On November, 13th 2020, the consortium obtained the resignation of Colin Dyer, president of the supervisory board but also its vice president and three other board members, at the end of this meeting Niel’s ally Leon Bressler was elected president of the supervisory Board.