Article II, Section 2 of the United States Constitutionwhich states that the President "shall have power to grant reprieves and pardons for offenses against the United States, except in cases of impeachment ..." meaning that he can pardon any such, known and unknown, to the date of the pardon.
You are correct. A president can pardon someone for crimes he/she has never been convicted or even accused of. That is exactly what Gerald Ford did to Richard Nixon in September 8, 1974
"Ford’s pardon ended any possibility of Nixon being prosecuted criminally. It covered more than obstruction of justice, however. It excused Nixon for 'all offenses against the United States which he, Richard Nixon, has committed or may have committed or taken part in' during the period from January 20, 1969, through August 9, 1974. In short, Ford’s pardon covered any crimes Nixon may have committed between the time he was sworn in for his first term and the day he resigned.
Blanket Pardons
Nor was this the first such pardon. The leading Supreme Court case is Ex parte Garland (1867). Justice Stephen J. Field, writing for the Court in a 5-4 decision, held that the President's pardoning power is ''unlimited,'' and ''It extends to every offense known to the law, and may be exercised at any time after its commission, either before legal proceedings are taken, or during their pendency, or after conviction and judgment.''
Constitution Allows Pardons Before Conviction
In the cited case Garland, an attorney took part in a rebellion against the United States Government. Congress passed a law on January 24th, 1865, which specified that no one would be allowed to practice law unless first taking an oath swearing, essentially, that he had never participated in any action against the United States. On July of 1875, Garland received a blanket pardon from President. Andrew Johnson and attempted to resume his legal practice. However he was denied because he could not take the oath required the act of January 24, 1865. Garland appealed to the Supreme Court and the following are excerpts from that case:
“The power [to pardon] thus conferred is unlimited, with the exception stated. It extends to every offence known to the law, and may be exercised at any time after its commission, either before legal proceedings are taken or during their pendency or after conviction and judgment. This power of the President is not subject to legislative control. Congress can neither limit the effect of his pardon nor exclude from its exercise any class of offenders. The benign prerogative of mercy reposed in him cannot be fettered by any legislative restrictions.”
“The pardon produced by the petitioner is a full pardon 'for all offences by him committed, arising from participation, direct or implied, in the Rebellion,' and is subject to certain conditions which have been complied with. The effect of this pardon is to relieve the petitioner from all penalties and disabilities attached to the offence of treason, committed by his participation in the Rebellion. So far as that offence is concerned, he is thus placed beyond the reach of punishment of any kind. But to exclude him, by reason of that offence, from continuing in the enjoyment of a previously acquired right is to enforce a punishment for that offence notwithstanding the pardon. If such exclusion can be effected by the exaction of an expurgatory oath covering the offence, the pardon may be avoided, and that accomplished indirectly which cannot be reached by direct legislation. It is not within the constitutional power of Congress thus to inflict punishment beyond the reach of executive clemency. From the petitioner, therefore, the oath required by the act of January 24th, 1865, could not be exacted even if that act were not subject to any other objection than the one thus stated."
"It follows, from the views expressed, that the prayer of the petitioner must be granted."
Ex Parte Garland