[10][11] Niel started early by developing telecommunication and data services and embarked at a young age on an entrepreneurial career soon after his father gave him a Sinclair ZX81 computer as a Christmas present when he was 15.

He has stated that, to avoid being arrested, he briefly did espionage for Direction de la surveillance du territoire, which allegedly included accessing the phone data of then-president François Mitterrand.

[14][15] 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.

[19] In 2012, he created Free Mobile, which offered unlimited voice calls, text and data at the lowest price of the market in France (€19.99 per month).

After restructuring its assets leading to the creation of the holding HoldCo, Niel did major maneuvers to reconquer the markets and strengthen its position in the capital of Iliad.

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

[31][32] Through NJJ, Niel became the largest shareholder of fixed line and mobile telecommunications services provider operator in Latin America, Millicom, in 2023.

[43] In March 2010, Niel cofounded with Jeremie Berrebi Kima Ventures,[44] a fund dedicated to invest in 50 to 100 startups a year everywhere in the world.

[49] In June 2017, Niel welcomed the French President Emmanuel Macron for the inauguration of Station F, a business incubator for startups located in Paris, known as the largest in the world.

[50] In April 2021, he became the first shareholder of the company Unieuro, which is the largest Italian retailer of consumer electronics and household appliances by number of outlets, with a network of 460 stores throughout Italy.

[51] In April 2023, Niel was part of an investor group that announced that it had bought a 7.5 per cent stake in Swiss asset manager GAM.

[52] In October 2023, Niel together with Rodolphe Saadé, Eric Schmidt, and other investors announced a €200 million investment in artificial intelligence which includes "the purchase of a supercomputer and the creation of a dedicated research laboratory," later named Kyutai.

[61][62] His book "Une Sacrée Envie de Foutre le Bordel" ("An Urge to Make a Mess"), co-written by former Paris deputy mayor Jean-Louis Missika [fr] was published in 2024 by Groupe Flammarion.

[63][64] In May 2004, Niel was indicted and detained for a month at the La Santé Prison for alleged procuring and misuse of company assets, which occurred in several sex shops in which he was a shareholder.

In August 2005, a nolle prosequi was pronounced for the procuring part of his indictment, while in October 2006, he was given a two years suspended prison sentence for the misuse of company assets.