There’s always a first time. The following, I believe, are all the presidential elections decided by third-parties:
1844 - Liberty Party defeats the Whigs and flips election to Democrats
1848 - Martin Van Buren (Free Soil party) defeats the Democrats and flips the election to the Whigs
1912 - Teddy Roosevelt defeats Taft and flips election to Woodrow Wilson
1992 and 1996 - Ross Perot defeats Republicans and flips elections to Bill Clinton
2000 - Ralph Nader defeats Al Gore and flips election to George W. Bush
TWO other presidential elections might be mentioned:
1856 - Fillmore (American/Whig candidate) helps defeat Fremont (Republican candidate) and helps election Buchanan (Democrat candidate). Filmore only flipped three states to the Democrats (CA, IL and NJ). So, although Buchanan was elected with less than a majority of the nationwide popular vote, he won enough states with a majority, that it is clear he would have been elected whether or not Filmore was in the race.
1968 - George Wallace almost defeats Richard Nixon and almost flips election to Hubert Humphrey. Although Nixon won the election, it turned out to be very close. Humphrey only carried four states with a majority of the popular vote. Nixon would have achieved a landslide if Wallace were not in the race.
What I think is interesting as well about the elections of 1852 and 1856 is that a third party (gop) was on the rise, and the whigs essentially went defunct.