If you went to an American city and asked random people on the street about what ended the institution of slavery in the United States, my guess is that the most frequent answer would be the Emancipation Proclamation. While Lincoln’s Proclamation was a major step toward the abolition of slavery, what actually ended slavery in the US was the 13th Amendment.
The Emancipation Proclamation actually consists of two documents, one published on 22nd September 1862, which announced that on 1st January 1863, “all persons held as slaves within any State, or designated part of a State, the people whereof shall then be in rebellion against the United States shall be then, thenceforward, and forever free”. It was followed by an executive order on 1st January 1863, which instructed the executive government of the US to free all slaves within the states and territories that were considered to be in rebellion.
This did not end the institution of slavery for two obvious reasons. First, there were still a number of slave states within the Union and the Proclamation did nothing to abolish slavery in those states. Second, you can’t free slaves that you don’t control. Lincoln of course did not regard the Confederacy as a legitimate government and still considered himself to have presidential authority over all the states, but in practical terms the CSA were a separate country and the Civil War was far from decided at that point in time.
So when it was issued, the Emancipation Proclamation freed slaves that the US government had no power to free, and did not free those slaves it had the power (if not the constitutional authority) to free. The only exception were slaves from Confederate states that the Union army had captured and held as “contraband”. As the war continued, more and more slaves were freed under the Proclamation as the Union army gained ground and many slaves in Confederate-controlled lands escaped from their masters and fled to the Union.
But the final abolition of slavery in the United States only came after the war, with the 13th Amendment, which declared that “Neither slavery nor involuntary servitude, except as a punishment for crime whereof the party shall have been duly convicted, shall exist within the United States, or any place subject to their jurisdiction.”