According to Harry Potter Wiki
Mnemone Radford was the witch who first developed Memory Modifying Charms. She became the first Ministry of Magic Obliviator, due to her skill with the charm.
When exactly did she do this?
The Ministry of Magic was founded in 1707. So, if Mnemone Radford was the first Ministry of Magic Obliviator, then it stands to reason she must have gained that title at the earliest in 1707 when the ministry was founded.
Except that she died a good 58 years before that date.
So there appears to be an inconsistency.
Perhaps someone involved was false-memory charmed!
This factoid seems to relate to one of the Wizard of the Month cards that appeared on JKR's original fan site. It notes that she developed "memory modifying charms" but doesn't outright state that she invented the memory charm, of which there are apparently several.
Note that although there's a discrepancy in her date of death (1649) and the formal foundation of the present Ministry of Magic (1707), there were apparently other bodies that governed the Magical world in Britain. It's certainly possible that she worked for the Wizard Council or an earlier Ministry and that this card is just over-simplifying her working history for the benefit of a Muggle audience or it may simply be a typo, given that wizards can live into their hundreds of years.
*Prior to 1707, the Wizards’ Council was the longest serving (though not the only) body to govern the magical community in Britain. After the imposition of the International Statute of Secrecy in 1692, however, the wizarding community needed a more highly structured, organised and more complex governing structure than they had hitherto used, to support, regulate and communicate with a community in hiding. Only witches and wizards who enjoyed the title of ‘Minister for Magic’ are included in this entry.