The Labour former prime minister, who represents Kirkcaldy and Cowdenbeath, made the comment as he took part in an education summit in the Gulf state of Qatar.
Mr Brown has in the past faced criticism over the frequency of his speeches in the Commons since returning to the backbenches.
And Tory MP Henry Smith said the latest "foreign jaunt" shows he "can't even be bothered to turn up to work" for his constituents.
Taking part in a panel discussion, Mr Brown was asked his views "as a politician", at which point he interjected "ex-politician".
He was reminded by the host - BBC presenter Mishal Husain - that he was still an MP, which he acknowledged.
Mr Smith said: "Crisis talks averted the loss of hundreds of jobs at Grangemouth last week, just a few miles from Gordon Brown's constituency.
"But where was the Labour ex-leader? He was sunning himself in Mexico, at a beachside hotel in Acapulco.
"Today, Gordon Brown joked that he is an 'ex-politician', whilst on another foreign jaunt. But he is still paid more than £65,000 a year to be an MP. Hard-pressed taxpayers are footing the bill.
"Labour wrecked our economy, and now Gordon Brown can't even be bothered to turn up for work."