Mather was Secretary of State during the Canisian invasion.
The Doctor met Mather after his election to the position of President while investigating phony time traveler Baskerville. At this point Mather was the only politician that lifelong spy Jonah Cosgrove really trusted, despite his pro-British stance. Despite this, Mather worked with the Doctor in thwarting Baskerville's prophecies. President Mather served sometime around 2010, when the US and Eurozone were rival blocs; during his time, the US and Eurozone had sent competing peacekeeping forces to North Africa. (PROSE: Trading Futures)
Behind the scenes
The Canisian invasion is a nod to Death Comes to Time, a non-canon story that was later implied to be an alternate timeline. (AUDIO: Zagreus) If the Canisians invaded at the same time as the webcast (2001 to 2002), that would make Mather the Secretary of State for George W Bush and a Republican. (Or, using the political timeline established by earlier books, Secretary of State for President Bruce Springsteen!)