The acquisition will allow Xaxis to provide its clients with an "enhanced suite of mobile performance capabilities".
The announcement comes on the same day that ZenithOptimedia predicted mobile internet to take a greater share of the market.
Mobile’s share is expected to increase by 7.4 percentage points, taking 12.7 per cent share of global adspend by 2017, the report said.
Brian Lesser, the global chief executive at Xaxis, said: "ActionX is one of the foremost players in mobile performance advertising, with industry-leading technology and a proven platform utilised by mobile-focused retailers and subscription brand.
"Today’s acquisition brings ActionX’s technology and expertise in-house, strengthening our mobile capabilities and supporting our broader strategy of developing new ways for brands to programmatically engage with consumers through proprietary media products."
ActionX was founded by Evan Schwartz and Scott Marshall, who will continue to run the business with the management team.
Schwartz said: "The combination of our mobile-first platform with the products, client reach and scale of Xaxis is extremely powerful.
"We are excited to continue to build our platform and grow our business as part of the Xaxis team."