The practices goes back to the colonial era. Here is Olivia Waxman at Time:
In the U.S., showing up in person to cast one’s ballot on Election Day has always been the standard way of exercising that fundamental right. But over the centuries, voting by mail has become an attractive alternative for many—thanks in large part to the influence of wartime necessity.
Even the scattered examples of absentee voting (the terms are often used interchangeably) that can be traced to the colonial era tend to fit the pattern: In 17th-century Massachusetts, men could vote from home if their homes were “vulnerable to Indian attack,” according to historian Alex Keyssar’s book The Right to Vote: The Contested History of Democracy in the United States, and the votes of some Continental Army soldiers were presented in writing “as if the men were present themselves” in Hollis, N.H., in 1775 during the American Revolution.
But it was during the Civil War that America first experimented with absentee voting on a large scale, as so many of the men who were eligible to vote were away from home fighting. During the 1864 presidential election—in which Republican incumbent President Abraham Lincoln defeated Democratic candidate George McClellan—Union soldiers voted in camps and field hospitals, under the supervision of clerks or state officials.
Read the rest here.