Morison left the Lions in 2011, but after mixed spells at Norwich City and Leeds United, the forward has rejoined his former club on a season-long loan deal.
The 29-year-old told London24: "Once I knew that they were interested and Leeds were prepared to do the deal, then it was a no-brainer.
"I did it myself. I have someone who helps me out with the financial side of it. It was just a few phone calls.
"I wasn't interested in waiting until the end of pre-season. I was happy to come to Millwall and play."
The Wales international scored 40 goals in 95 appearances during his previous spell at the Championship outfit.