Negotiations are ongoing but the England Under-21 international is expected to join the Lions on a 93-day loan in time for their Sky Bet Championship fixture against Derby at The Den on Saturday.
Former Millwall manager Kenny Jackett tried to sign Waghorn on loan last year while current Lions boss Steve Lomas renewed the London club's interest in the 23-year-old during the summer but an agreement could not be reached.
However, it appears Millwall's efforts have finally borne fruit and a they hope to announce the signing on either Thursday or Friday.
Waghorn, in the final year of his contract, was brought to Leicester from Sunderland by Paulo Sousa in a £3million deal in the summer of 2010 following a season-long loan spell with the Foxes.
He has been unable to hold down a first-team place following the arrival of New Zealand international Chris Wood in January.
Millwall have problems up front with Jermaine Easter (hamstring), Andy Keogh (groin) and Steve Morison (double hernia operation) all out injured.