Maryland was first recognized as a "Free State" on November 1, 1864.   On that date, the Maryland Constitution of 1864 took effect.   By its provisions, slavery within the State's borders was abolished, and Maryland, indeed, became a free state.

March 1 to June 1, 1860 - In these three months, 136 slaves were manumitted by their owners in Frederick County.   This was in advance of a new state law which took effect on June 1st, 1860, which prohibited the freeing of slaves.
[from John W. Ashbury's "History Moments"]

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