In 1988 he joined Cherepovetskiy Metallurgical Plant after graduation from Leningrad Engineering-Economical Institut as an Economist. By 1992 he became Finance Director, shortly before the company was privatised. When the plant's elderly Director instructed him to acquire shares to keep them out of the hands of an outsider, he formed two investment funds and, by buying up workers' shares, built a major stake in the factory. In 1996 he was appointed as CEO of the company. He went on to build a conglomerate, Severstal, acquiring steel, coal and mining companies.
Mordashov and his first wife Elena (sometimes Yelena) have a son, Ilya, and divorced in 1996. In 2001, it was revealed in a St Petersburg divorce court that he was paying only $620 a month to support his ex-wife and son, when she sued him for half of his pre-divorce property but lost the case and incurred large fees. The case was eventually settled after she took it to the European Court of Human Rights.
By 2001, Mordashov had married his second wife, also Elena, and had had two more sons.
Between 2004 and 2014, Mordashov and Severstal invested up to $3 billion in both the Dearborn and Columbus plants. In 2011 he sold a number of US assets in Ohio, Maryland and West Virginia to the Renco Group, owned by industrialist Ira Rennert. By 2014 all of Severstal's US Investments had been sold for a total of $2.3 billion.
His company Severstal planned a multibillion-dollar project in joint venture with POSCO in eastern India. After ten years of not getting environmental and land clearances, his company opted out in 2008.
In 2011, Mordashov purchased 25% of the shares in steam turbine manufacturer, Power Machines, from Siemens. In 2013, with partner Yuri Kovalchuk, Mordashov bought 50% of Tele2 Russia, the country's 4th largest mobile phone operator.
In 2015, Mordashov stepped down as CEO, appointing former COO Vadim Larin, in his place. Mordashov would be appointed as chairman. The changes were confirmed by Severstal.