Not quite correct. Mitt's great-great grandfather, Miles Park Romney, moved to the polygamous LDS colonies in Mexico in 1884, before the 1890 Manifesto (which wasn't enforced by the LDS church; hence the 1904 Manifesto and the Reed Smoot hearings), but after polygamy was illegal.
Miles Park Romney was a polygamist. So was another of Mitt's great-great grandfathers, Helaman Pratt (he's from the line related the the "celestial veggie" guy, above).
Gaskil Romney, Mitt's grandfather, fled Mexico during the Mexican Revolution in 1912 (Pancho Villa wasn't a nice guy) and returned to the U.S., but Gaskil wasn't a polygamist.
Thank you for the corrections. You are right.
You know what UK Daily Mail dig up dirt on Mitt Romeny Mexican relatives one of them still living in Mexico