I see that the wording on his Wikipedia page has the ambiguous phrase "had
the first implementation of FTP", which has been flagged as needing
clarification, so I intend to provide it.
In both this interview:
https://conservancy.umn.edu/bitstream/handle/11299/113899/oh403dlm.pdf
... and this video recording of Mills himself giving a lecture at UDel:
https://youtu.be/08jBmCvxkv4?t=428
... it's quite clear that it's literally true - he authored, compiled,
installed, implemented, and tested the very first (and apparently second)
FTP server.
It may be impossible to provide hard evidence. From RFC 354 it seems to me that the protocol took on a recognisable shape around July 1972 and from RFC 414 it seems to me that there were a number of implementations by November 1972, and unfortunately Dave Mills is not mentioned. His recollection may well be correct, but finding proof he was the first in a 4 months time slot 50+ years ago may be too ambitious.
https://www.ietf.org/rfc/rfc354.txt
https://www.ietf.org/rfc/rfc414.txt
Maybe the internet history list can shed some more light on the matter:
https://elists.isoc.org/mailman/listinfo/internet-history