We know that Michael "Mike" Ehrmantraut from Breaking Bad was a former Philadelphia police officer who worked as the Head of Corporate Security at Los Pollos Hermanos.
How did he end up working with Gus?
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This is explained in the series Better Call Saul, the spin-off from Breaking Bad.
Mike unintentionally helped Gus' business. His efficiency, professionalism and meticulous behaviour attracted Gus' attention.
Don't read further if you plan to watch the series.
Back in 1999, Gus and Hector Salamanca were both working for Don Eladio. When Gus started to make more money than Hector, a rivalry started between them. Years later, Mike (who did not know Gus) hijacked one of Hector's shipments. This attracted attention from Gus, due to his rivalry with Hector. Gus tracked Mike down and told him to stop trying to kill Hector, but that he has no issue with Mike messing with Hector's business.