The earliest known use and mention of a rear-view mirror is in the 1906 book The Woman and the Car which noted that women should "carry a little hand-mirror in a convenient place when driving" so they may "hold the mirror aloft from time to time in order to see behind while driving in traffic" therefore inventing the rear view mirror before it was introduced by manufacturers in 1914.
The earliest known rear-view mirror mounted on a motor vehicle was in Ray Harroun's Marmon racecar at the inaugural Indianapolis 500 race in 1911.
According to Al Binder of Ward's Auto World: As per the custom of the day, all cars except Harroun's carried riding mechanics who, among other things, helped the driver keep track of other vehicles during the race. Unable to find a mechanic to ride with him, Harroun installed a mirror on his car so he could view what was happening behind him and be alert to any cars overtaking him.
Although Harroun's use is the first known use of such a mirror on a motor vehicle, Harroun himself claimed he got the idea from seeing a mirror used for the same purpose on a horse-drawn vehicle in 1904. The invention seems to have worked — Harroun won the race, netting a $14,250 prize, equivalent to about $280,000 today.
However, the rear-view mirror had to wait for Elmer Berger, the man usually credited with inventing the rear-view mirror, to first develop them for street use. (info from Wikipedia)