The appointment came several months after McCann took part in a multi-agency pitch initially intended for the regional business only.
But when McCann's recommendations were put before Siemens' worldwide board in Germany, the IPG agency was given the nod.
Publicis Germany was the global incumbent.
Siemens corporate communications vice-president Gunter Baumgartner said: "McCann Thailand developed unique and powerful solutions for our marketing that we felt had global resonance."
Mark Ingrouille, McCann Thailand president and chief executive officer, said: "This is a big, top-down branding initiative, which would trickle down to regional and local levels. Because Siemens projects involve large sums of money, spanning many decades, there is a need to establish confidence among governments and big business."
He added that an upcoming campaign would be print-led, with some corporate TVCs airing on BBC World and CNN.
A global roadshow highlighting Siemens' power transmission and distribution businesses is also planned.