1777
Vermont is the first of the colonies to abolish slavery when it does so in its constitution dated July 8, 1777. Chapter 1 of the Constitution, subtitled "A DECLARATION OF THE RIGHTS OF THE INHABITANTS OF THE STATE OF VERMONT" states: "THAT all men are born equally free and independent, and have certain natural, inherent and unalienable rights, amongst which are the enjoying and defending life and liberty; acquiring, possessing and protecting property, and pursuing and obtaining happiness and safety. Therefore, no male person, born in this country, or brought from over sea, ought to be holden by law, to serve any person, as a servant, slave or apprentice, after he arrives to the age of twenty-one Years, nor female, in like manner, after she arrives to the age of eighteen years, unless they are bound by their own consent, after they arrive to such age, or bound by law, for the payment of debts, damages, fines, costs, or the like."

1780
The Pennsylvania legislature passes "An Act for the Gradual Abolition of Slavery." Significantly, the act freed the future children of slaves and required that slaves born before the act remained enslaved for the rest of their life. This act became the model for other northern states. The last slaves in Pennsylvania were freed in 1847.

1783
In Massachusetts, the Supreme Court ruled slavery unconstitutional based on its own 1780 state constitution. As a result of this decision, all slaves of Massachusetts were immediately freed.
The 1783, the New Hampshire state constitution declared "all men are born equal and independent," with natural rights, "among which are enjoying and defending life and liberty." 

1784
The abolition forces in the state Congress in Connecticut presented a bill for gradual emancipation as part of a general statute codifying race relations. As an afterthought, it provided that black and mulatto children born after March 1 would become free at age 25. The strategy worked, and the bill passed without opposition. An act of 1797 reduced that age to 21, bringing slavery in line with apprenticeship, though obviously slavery was not voluntary and slaves did not receive money, clothes and professional standing at the end of their servitude. As in other Northern states, gradual emancipation freed no slaves at once. It simply set up slavery for a long-term natural death. Connecticut finally abolished slavery entirely in 1848.
Rhode Island begins the gradual abolition of slavery. The Act of the Rhode Island State Congress declared all children of slaves born after March 1 were to be "apprentices," the girls became free at 18 and the boys became free at 21. As with other Northern instances of gradual emancipation, this gave slaveowners many years of service to recoup the cost of raising the children. The last slaves were counted in the 1840 census.

1787
The Northwest Ordinance of 1787 was passed by the United States Congress, which outlawed the establishment of slavery in the Northwest Territories of the United States. In article 6 of the legislation it states, "There shall be neither slavery nor involuntary servitude in the said territory, otherwise than in the punishment of crimes whereof the party shall have been duly convicted."

1799
New York passes a gradual emancipation act freeing future children of slaves, and all slaves in 1827 with a universal emancipation bill. Slaves already in servitude before July 4, 1799, remained slaves for life, though they were reclassified as "indentured servants." The law sidestepped all question of legal and civil rights, thus avoiding the objections that had blocked the earlier bill.

1804
In 1804, the New Jersey Legislature passed "An Act for the Gradual Abolition of Slavery." It provided that females born of slave parents after July 4, 1804, would be free upon reaching 21 years of age, and males upon reaching 25. New Jersey slaveowners had the option to sell their human property into states that still allowed slaveholding, or into long indentures in Pennsylvania, until an 1818 law that forbid "the exportation of slaves or servants of color."

1863
A pro-Union coalition from Western Virginia requests statehood from Abraham Lincoln in the midst of the Civil War. Because its mother state, Virginia, had seceded from the Union and was in active rebellion against the United States, Lincoln insisted that the new state could only be admitted to the Union if it did so as a free state. As a result of this compromise, West Virginia became a free state on June 20, 1863.

1864
Even after the Emancipation Proclamation was issued in January of 1863, Lincoln persistently advocated the process of state abolition. He understood that the Emancipation Proclamation was limited in its scope. Additionally if he lost the election of 1864 to a Democratic pro-slavery candidate, it would not be certain that slavery would be completely abolished. On March 4, 1864, Arkansas adopts a new state Constitution using the same language of the Northwest Ordinance. It was the first state to abolish slavery since New Jersey in 1804.
Lincoln lobbied Major General Banks in late 1863 to work to permanently abolish slavery in Louisiana. On September 4, 1864, Louisiana followed the lead of Arkansas and ratified a new state Constitution that abolished slavery in the entire state. Article 1 of the new constitution stated, "Slavery and involuntary servitude, except as a punishment for crime, whereof the party shall have been duly convicted, are hereby forever abolished and prohibited throughout the State."
Abraham Lincoln wrote and tried to convince Maryland to abolish slavery in 1864. Maryland abolished slavery through the ratification of a referendum on October 12, 1864. Article 24 of the referendum explained, "That hereafter, in this State, there shall be neither slavery nor involuntary servitude, except in punishment of crime, whereof the party shall have been duly convicted; and all persons held to service or labor as slaves are hereby declared free."

1865
On January 11, 1865, Missouri passed "An Ordinance Abolishing Slavery in Missouri." It is important to keep in mind that this was passed before the end of the war and prior to the proposal of the Thirteenth Amendment to abolish slavery in all states.
A dispatch from Nashville on January 14, 1865 read, "The Tennessee State Convention have unanimously passed a resolution declaring slavery forever abolished, and prohibiting it throughout the State. The convention also pasted a resolution prohibiting the Legislature from recognizing property in man, and forbidding it from requiring compensation to be made to the owners of slaves." 
Passed by Congress on January 31, 1865, and ratified on December 6, 1865, the 13th amendment abolished slavery in the United States and provides 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."