James will be responsible for leading the agency's online strategy across its full client base, which includes brands such as O2, EDF Energy and Skoda.
In recent months, AIS has launched a number of high-profile digital campaigns, including the '3D Rugby' work for O2, and the 'Get some cider inside ya' work for Magners.
During his time at Partners Andrews Aldridge, James oversaw the digital planning across accounts including Vodafone, Rolls Royce and GlaxoSmithKline.
He joined the agency in January last year from Star Creative, where he had worked as digital strategy director.
Jon Ingall, the managing partner of AIS, said: "Lachlan's experience in heavyweight strategic online projects for major brands will be a real advantage to us, and will certainly boost our digital planning offering."