Merriam Webster
Pathological
Main Entry: path·o·log·i·cal
Pronunciation: \ˌpa-thə-ˈlä-ji-kəl\
Variant(s): also path·o·log·ic \-jik\
Function: adjective
Date: 1688
1: of or relating to pathology
2: altered or caused by disease ; also : indicative of disease
3: being such to a degree that is extreme, excessive, or markedly abnormal <a pathological liar> <pathological fear>
— path·o·log·i·cal·ly \-ji-k(ə-)lē\ adverb
He says he suffered from "some severely undiagnosed mental illness".
That's more pathological lying.