Stock ended the day at $25, making founder Paul Palmieri a paper $185m.
It was all a bit of a triumph for the ad network, which earlier speculation had suggested could raise $818 million from the stock sale, but which actually expects to raise more than $132 million.
Millennial has grown fast over the six years since its formation, but needed to go for big funds to keep up with well-heeled rivals such as Google-backed AdMob, Apple-backed Quattro, Microsoft and InMobi, which recently opened up a $200m credit line with Softbank.
The firm has already raised $65 million from venture capital firms. It has yet to report a profit, but did succeed in reducing its losses from $7.1 million in 2010 to $287,000 last year.
IDC estimates that Millennial has a 17 per cent share of the US mobile ad market. The firm is less well established outside of the States, but is currently growing its teams in London and Singapore.