Mr Sondhi's driver was said to be in a serious condition, while an aide suffered minor injuries.
The attack comes just days after protests by the yellow-shirts' sworn enemies, the pro-Thaksin red-shirts, came to an end.
The red-shirts held large rallies in the streets of Bangkok, forcing the cancellation of a high-profile Asian summit and clashing with security forces and local residents. Two people died in the violence.
But so far it is unclear who was behind the attack on Mr Sondhi.
The BBC's Alastair Leithead in Bangkok says the media mogul and businessman has many enemies - among the red-shirts, but also in the police, the army and the current government.
The attack will increase tension between reds and yellows and lead to greater factionalism in an already deeply divided country, our correspondent says.
The attack on Mr Sondhi came at dawn on Friday, as he was travelling to record a programme at his television station.
Sondhi Limthongkul opposed former prime minister Thaksin Shinawatra
"At least two attackers followed Sondhi's car, overtook it and sprayed it with about 100 rounds of gunfire from AK-47 and M-16s," said local police commander Colonel King Kwaengwisatchaicharn, adding that an investigation was under way.
In the wake of the attack, security was increased around the current Prime Minister Abhisit Vejjajiva, who is working out of an undisclosed location because of fears for his safety.
Mr Vejjajiva said a state of emergency imposed on Sunday had sent a "clear signal" that the "use of violence and intimidation" to try to topple a government would not be allowed.
"Now what I would like is to appeal to everybody... that anybody who has grievances or causes that they think are valid they can express themselves and even negotiate with us to achieve those ends," he said in an interview with the BBC.
The largely peaceful demonstrations that paralysed parts of Bangkok turned violent earlier this week.
Protest leaders called off the action amid a major military crackdown to quell the riots.
Thailand's Prime Minister Abhisit Vejjajiva discusses the anti-government protests
The red shirts took to the streets demanding that Prime Minister Abhisit step down, and fresh elections to be held.
They say that he was illegally installed by parliament in December after courts ousted the government led by Mr Thaksin's allies, and dissolved their parties.
The red shirts have expressed anger over the detention of several protest leaders in recent days, while Mr Sondhi and his allies were never prosecuted for their political action.
Last year, the yellow shirts occupied Government House for three months and seized Bangkok's two airports for a week, stranding hundreds of thousands of travellers.
Meanwhile, Mr Thaksin - who has been calling for a popular uprising against the government - has appealed to the widely-revered Thai king, Bhumibol Adulyadej, to intervene to end the country's political crisis.
Thailand annulled Mr Thaksin's passport after his supporters forced the cancellation of a high-profile Asian summit last weekend.
Mr Thaksin faces two years in jail if he returns to Thailand, after being sentenced in absentia for abuses of power when he was in office.
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