American-born Colvin was a renowned reporter who had covered countless conflicts over 30 years and wore a distinctive eye patch after she was wounded in Sri Lanka.
She was voted Foreign Correspondent of the Year in the 2010 British Press Awards and had issued powerful reports from Homs until hours before her death.
Ochlik, who was also killed, was a 28-year-old photographer represented by the IP3 agency, which he co-founded in Paris, who quit his studies aged 20 to report on Haiti and has since covered many of the recent upheavals in the Arab world.
They were among at least 26 people killed Wednesday as Syrian forces pounded Homs, activists said, as calls mounted for a truce to allow in humanitarian aid to the shattered city.
Colvin told BBC television on Tuesday: "I watched a little baby die today. Absolutely horrific... His little tummy just kept heaving until he died."
The pair were killed in a bombardment of the Baba Amr area, French Culture Minister Frederic Mitterrand said.
The French daily Le Figaro said one of its reporters, Edith Bouvier, had been among three journalists wounded in the same incident, while Rupert Murdoch, owner of the Sunday Times' parent company, said the newspaper's photojournalist Paul Conroy was injured.
Separately, Syrian citizen journalist Rami al-Sayyed, who provided live footage online from Baba Amr, was killed late Friday when a rocket hit a car in which he was travelling, activist Hadi Abdullah said.
David Cameron paid tribute to Colvin, saying her death in Syria showed the risks journalists face in exposing the truth.
"This is a desperately sad reminder of the risks that journalists take to inform the world of what is happening and the dreadful events in Syria, and our thoughts should be with her family and with her friends," Cameron told parliament.
Describing the journalist as "talented and respected", Cameron took the unusual step of mentioning her at the start of his weekly parliamentary questions after the names of service personnel killed in Afghanistan.
Media tycoon and Sunday Times owner Rupert Murdoch hailed Colvin as one of the best foreign correspondents of her generation.
"It is with great sadness that I have learned of the death of Marie Colvin, one of the most outstanding foreign correspondents of her generation, who was killed in Homs in Syria today while reporting for The Sunday Times," Murdoch said in an email to staff.
"She was a victim of a shell attack by the Syrian army on a building that had been turned into an impromptu press centre by the rebels.
"Our photographer, Paul Conroy, was with her and is believed to have been injured. We are doing all we can in the face of shelling and sniper fire to get him to safety and to recover Marie's body."
The latest violence came after government forces killed at least 68 across the country and amid international calls for a truce.
Activist Omar Shaker told AFP that two were killed and three others wounded when a shell crashed into a makeshift media centre set up by anti-regime militants. The area remained the target of random shelling, blocking attempts to remove the bodies, he said.
Le Figaro said Edith Bouvier suffered leg wounds.
"I received two calls from Homs this morning to tell me that Edith was wounded in the legs. We're trying to organise her evacuation," foreign editor Philippe Gelie told AFP in Paris.
France on Wednesday demanded access to the victims of the attack, and summoned Syria's envoy to Paris.
French President Nicolas Sarkozy said the killing of the two journalists showed that "this regime must go".
The Syrian opposition said the journalists' deaths were likely the fault of the regime.
"Homs is a very, very dangerous place," Bassma Kodmani, spokeswoman for the Syrian National Council, the most representative Syrian opposition umbrella group, told reporters in Paris.
"I see no reason why opposition members would shoot at journalists," she said. "It is, therefore, most probably related to the regime."
The Sunday Times' editor John Witherow said he was in "great shock" at Colvin's death.
"Marie was an extraordinary figure in the life of The Sunday Times, driven by a passion to cover wars in the belief that what she did mattered," he said in a statement.
"But she was much more than a war reporter. She was a woman with a tremendous joie de vivre, full of humour and mischief and surrounded by a large circle of friends, all of whom feared the consequences of her bravery."
Witherow said reports suggested Conroy "is not too seriously hurt".
