The house she was staying in was shelled and the veteran reporter was killed by a rocket as she tried to escape, along with French photographer Remi Ochlik.
Three other foreign journalists were also wounded when over 10 rockets hit the house, including British photographer Paul Conroy and Edith Bouvier, a French reporter working for Le Figaro.
A female American journalist who has not been named was said to be “in a very serious condition”.
Colvin wore a trademark black eye patch since she lost an eye while working in Sri Lanka in 2001 and was seen as Britain’s foremost front-line war reporter.
The two were reported to have been killed after a shell crashed into a makeshift media centre, set up by anti-regime activists in the Baba Amr district of Homs.
The area has been under a sustained bombardment from government forces since February 3, killing several hundred people.
In a BBC broadcast from Syria yesterday, Colvin described the bloodshed.
She said: “I watched a little baby die today. Absolutely horrific. There is just shells, rockets and tank fire pouring into civilian areas of this city and it is just unrelenting.”
In the Sunday Times last weekend, Colvin wrote that the citizens of Homs were “waiting for a massacre”.
She wrote: “The scale of human tragedy in the city is immense. The inhabitants are living in terror. Almost every family seems to have suffered the death or injury of a loved one…
“On the lips of everyone was the question: ‘Why have we been abandoned by the world?'”
Colvin’s editor, John Witherow, spoke of his “great shock” at her death, and described her as “an extraordinary figure in the life of the Sunday Times”, who would be “sorely missed”.
Syria banned almost all foreign journalists from the country at the start of the uprising in March 2011, but has begun allowing short-term visas for a limited number of journalists, who are allowed to move around accompanied by government minders.