The Northern Ireland international – who spent the second half of last season at Rangers – has made two starts for Neil Harris's side so far.
Making two substitute appearance as well, the 24-year-old has done enough to earn a longer stay in London.
He had been due to return to Newcastle – for whom he has made 32 appearances including 13 starts – in November.
But Steve McClaren has allowed him to remain with Millwall for the sake of enjoying regular first-team football over the next few months.
Ferguson was sidelined for much of last term with a knee ligament injury and his only club outings came for Rangers in the Scottish Premiership play-offs in May.