After graduating (1970) he served as a trainee at the mill, and in 1976 he opened his own steel mill in Indonesia. He spent more than a decade learning how to run it efficiently.

In 1989 Mittal purchased the beleaguered state-owned steel works in Trinidad and Tobago, which had been losing huge sums of money. A year later that facility had doubled its output and had become profitable. He used a similar formula for success in a series of acquisitions all around the world, purchasing failing (mostly state-run) outfits and sending in special management teams to reorganize the businesses.

Mittal’s business philosophy emphasized consolidation in an industry that had become weak and fragmented. Although demand for steel remained high, smaller steel companies had been unable to strike competitive deals with their major clients, notably automakers and appliance manufacturers. Mittal’s company, however, controlled about 40 percent of the American market for the flat-rolled steel used to make cars, which allowed the giant steelmaker to negotiate more favourable prices.

In 2004 Mittal merged his companies, Ispat International and LNM Holdings, and acquired Ohio-based International Steel Group. The newly created company, Mittal Steel Co. NV, emerged from the deal as the world’s largest steelmaker. Two years later Mittal oversaw another merger when Mittal Steel joined with Arcelor to form ArcelorMittal.

Benefiting from an uptick in steel, the company reported double-digit growth in net profit to $5.1 billion in 2018 on revenue of $76 billion.
ArcelorMittal completed the $2.1 billion acquisition of Italy's loss-making steel group Ilva in November 2018.
In 2018, his $5.9 billion bid for bankrupt Essar Steel, once controlled by billionaires Shashi and Ravi Ruia, was accepted by Essar's creditors.

Accoridng to britannica.com and forbes.com