That was in 1972. She is still in Japan. Her hair has turned grey from the cold climate, long working hours and pervading loneliness of a Filipina in a country that does not welcome foreigners. 'I am too old to get a husband now,' she says. 'But at least my brothers and sisters have got a good education.'
Ms Mercado is one of hundreds of thousands of Filipinos who have gone overseas to earn a living and support relatives back home. She is not ashamed of what she does - in fact she is quite proud of helping the rest of her family.
But her government is ashamed on her behalf. According to 'Philippines 2000', the ambitious new vision for the future propounded by President Fidel Ramos, these workers will all return home by the end of the century. Mr Ramos hopes that by then the domestic economy will offer enough jobs for the entire population.
Mr Ramos has already made a start in deregulating and opening up the Philippine economy, which lags painfully behind the rest of South-east Asia. But for ordinary people like Ms Mercado, long accustomed to big promises and no action from their politicians, the Philippines 2000 ideal still sounds like a pipedream.
According to the government's Philippines Overseas Employers' Association, 686,461 Filipinos were legally registered as overseas contract workers in 1992. If illegal workers were included, the total would be well over a million, out of the country's population of 62 million. About 134,000 are seamen - 16 per cent of the total number of seamen in the world. Others work as maids, singers, construction workers and hostesses in bars from Saudi Arabia to Australia.
They send back an estimated pounds 1.4bn per year to the Philippines: equivalent to about 15 per cent of the national budget.
Salary discrepancies are so high that it is not uncommon to find maids in Hong Kong who have qualified as dentists or accountants back in Manila. But working overseas is not without its risks. Although Filipinos are generally English-speaking, well-educated and cheerful by nature, they are frequently subjected to exploitation by their foreign employers and by employment agencies.
Ms Mercado earns 150,000 yen (pounds 880) a month working as a maid for an embassy in Tokyo. She lives in a tiny one-room flat, works privately on her weekly day off to cover her food costs and sends as much of her salary home as she can. When she first came to Tokyo, she had to pay 20 per cent of her earnings to the agent who found her the job, but now she has struck out on her own. She is treated relatively well by the embassy.
But she tells horror stories about what has happened to younger women who came to Japan to work as singers or hostesses in bars and were forced to work as prostitutes.
At Christmas, President Ramos went to Manila International Airport to welcome home some of the Filipino diaspora of foreign workers returning for the holiday period. He called them 'modern-day heroes'. Sad for a country when its heroes have to spend their best, most productive years away from home.