The 26-year-old, originally from Northern Ireland, apparently came with excellent qualifications.
As the national daily France Soir reports, because the local education authority was particularly short on German teachers, it immediately offered him a job under contract at two of its schools; the Pierre-Gilles-de-Gennes lycée and the Maria-Borrely collège.
He began on January 3. But not all was as it at first appeared.
Cloth embroidered by a schizophrenia sufferer (from Wikpedia, author - cometstarmoon)
"We had no reason to complain about his behaviour and I never heard the slightest negative comment about him from his colleagues, pupils or parents," Pascale Garrec the director of the lycée is quoted as saying in the regional daily Midi Libre.
"It was during a conversation outside of the professional context that I became concerned over some of the 'peculiarities' about comments he made."
Among them were claims made by Mawhinney that he was a secret service agent working for Scotland Yard, and that led Garrec to alert the local police.
His behaviour in the classroom was also somewhat unusual according to pupils who spoke to another regional daily La Provence, and some of them found him "weird".
"He didn't seem to know the rules of German grammar," one pupil told the paper.
"When we asked him a question, he wouldn't reply immediately and instead would give us the answers the next day after having searched the Internet."
Another commented on the teachers apparent "normality" inside the classroom but odd habit of "putting on his gloves to open and close the door so as not to leave fingerprints."
Investigations revealed that the man described as "discreet" had in fact escaped from a clinic in the Northern Ireland capital Belfast in 2008, where he was being treated for schizophrenia after a knife attack on a man the previous year.
Mawhinney has been fired from his post and is being held in a psychiatric unit in the town awaiting his return to Belfast.