Do the Mormons follow a false prophet?

Yes. Joseph Smith, the first “prophet” and the founder of the Church of Jesus Christ of Latter-day Saints (more commonly known as the Mormons), taught things that contradict the Bible. If people say they are prophets but teach things that contradict the Bible, then they are false prophets. The prophet Isaiah says that if such people don’t speak  according to ‘the teaching and…the testimony’ (i.e., God’s previously revealed word), then ‘there is no light or truth in them’ (Isa. 8:20, NLT).

Two examples of Joseph Smith’s false teachings are:

• God was once a human¹ (but the Bible says that God has always been God and, unlike humans, has always existed:e.g., Ps. 90:2)
• When people die they continue to live as spirits² (but the Bible says that when people die they cease to exist, not knowing anything, unless they are resurrected when Jesus returns: e.g., Eccl. 9:5; 1 Cor. 15)

Also see

• ‘Does the Book of Mormon contradict the Bible?‘ on this site
• Failed Prophecies of Joseph Smith on Mormons in Transition
• Bible teaching about God: God Our Father on Christadelphia World Wide
• Bible teaching about death: After Death – What? on Christadelphia World Wide

Notes

1. See Smith’s “King Follett discourse” and his sermon recorded in chapter 23 of History of The Church of Jesus Christ of Latter-day Saints Vol. 6.

2. See, e.g., Doctrine and Covenant 138.11ff.