Mah was a member of Parliament (MP) since 1988 and he was Cabinet Minister from 1991 to 2011. Assuming an average take-home of a million dollars per year, his government pay alone could easily add up to $20+ millions, without accounting for interest income and returns from other investments. So $28 million is not a big number for the elite crowd.
GSH's original core business was in distribution (Apple, Tamron, Fujifilm, Corum, Noritsu, etc) - its corporate profile said it was established as distributors of IT, photographic, timepiece and healthcare products - until it diversified into property development in 2012. That's the year when Mah was listed as the sixth largest shareholder in the annual report, with a holding of 165,000,000 shares. A year later, his shareholding jumped to 365,575,000 or $28 million based on a share price of 7.7 cents.
The interesting bit is GSH was trading at around 1 cent, not more than 2 cents per share in 2012. Assuming Mah paid 1 cent per share, his total buy-in - there was a 1-for-1 rights issue in May 2013 - could have been as low as $4 million. Now, that's a foresight to die for.
Mah was National Development Minister from 1999 to 2011. Imagine if he had gone into private sector sooner, he could be one of Singapore's many billionaires. Maybe he already is. Lee Kuan Yew did say his ministers could make much, much more in the private sector.