May 17 2012
Who was the last mainstream Mormon polygamist prophet?
Heber J. Grant had three wives, married the first in 1877, and the other two in 1884. The church ended polygamy in 1890 (http://en.wikipedia.org/wiki/189…), but didn’t annul existing marriages or ask people to abandon their families, so Grant lived with his wives all their lives–but in those days that meant two died in 1893 and 1908, and the third died after him, in 1952. So the last mainstream Mormon polygamist prophet ceased being a polygamist as it were in 1908.
See http://en.wikipedia.org/wiki/Lis…
So the church was founded in 1830, so basically 2/3 of its existence there hasn’t been any polygamy, and even when it was on it was never a very high percentage of church members. I read once that there was never a time in history where the majority of polygamists in the US were Mormon, and if you count mistresses and abandoned wives nationally into the equation, it was never 1% of national polygamy. Net effect: most Mormons today have never met a polygamist and don’t know any more than you do about it. (They’re more likely to have read a piece of historical fiction on the topic, but less likely to have seen Big Love and far less likely to have had an affair–so it’s up to you how to do the math… 
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Now that I’ve said all that people will jump in and say there was unofficial polygamy going on for a while after 1890, since when rescinding a doctrine of a church everyone will go through the psychological process of thinking “if it’s divine, it’s eternal, so this change must have been show to get the haters off our backs, and God wants us to keep doing it in secret.” That’s all very reasonable and valid, and as I said went on for a short period. But by 1904 the church began excommunicating members who had anything to do with in these ceremonies, and it was never church policy anyway (and it definitely doesn’t fall within your question of “who was the last mainstream Mormon polygamist prophet.”)
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This post was in answer to a reader-submitted question at (http://www.quora.com/The-Church-of-Jesus-Christ-of-Latter-day-Saints-the-Mormons/Who-was-the-last-mainstream-Mormon-polygamist-prophet/answer/Colin-Jensen). If you have comments, or if there is a topic you would like Colin to address, please comment below!