For one reason or another, this is a relatively common myth. The notion that Ford Motor Company owns Cummins could be derived from the fact that Ford was a minority shareholder of Cummins' stock in the past, or that Cummins had previously supplied engines for Ford. Yes, for many years you could purchase a brand new Ford with a factory installed Cummins diesel - so long as you were in the market for one of the company's F650/F750 medium duty trucks.
Ford had offered Cummins diesels in their medium duty platforms side-by-side with the Power Stroke and even a Caterpillar option for multiple years. In fact, the 6.7L Cummins Turbodiesel was the only available diesel engine in the 2011-2015 model year F650/F750 (Ford did not initially offer their own 6.7L Power Stroke). Ready for a curve ball? Allison transmissions are also offered in the F650/F750 models!
Cummins set the record straight in their May 2013 newsletter, outlining that Ford Motor Company had purchased 10.8% of the diesel engine manufacturer in 1990. In 1997, those shares were bought back, and as such Ford has not owned a stake in Cummins since. Think of it as an investment for Ford, rather than a strategic partnership. Beyond the medium duty market, there are no additional ties between Ford and Cummins. If you were told that Ford owns or has owned Cummins, your source was blatantly incorrect.
Source: Cummins Inc., cummins.com