I loathe the team, of course, but isn't Man U owned by Yanks?
Yeah, the Glazers, but they've not put much money into the club for quite a few years and the team has suffered. Ferguson had managed them for 20+ years, and when he retired last summer, he left a team which had been over performing massively. The new manager couldn't get the same out of them, and they were struggling to make a top 4 finish this season. The owners did plough some cash in during January, but it might not be enough to get them Champions League football next season.
As a group of owners, they've done OK, but not great. Same with Randy Lerner who owned Villa, they just paid huge transfers and wages, resulting in the club being massively overstretched. They've had to sell almost the entire team and rebuild with younger cheaper talent instead.
Currently, the most successful US-owned club would be Liverpool, who have slowly built a team by investing wisely, and are on course to do very well this season.