It is illegal to criticise Mugabe
She was imprisoned last week on accusations of violating Zimbabwe's new media laws.
The controversial regulations outlaw criticism of President Robert Mugabe - and she could have faced up to five years in prison had she been found to be flouting them.
Ms Thornycroft, 57, was arrested last Wednesday in the eastern resort town of Chimanimani, while investigating reports of violence against opposition activists.
The next day she was transferred from Chimanimani to a jail cell in the regional capital, Mutare.
The editor of the Daily Telegraph, Charles Moore, had described the allegations against her as "ludicrous".
One of her solicitors, Tatiwanashe Kujinga, told Reuters news agency that Judge Mohamed Adams issued the order for Ms Thornycroft's release on Sunday.
Mr Kujinga maintained she had never been formally charged and on that basis the High Court should always have ruled she should be released.
Widowed
He added: "It was an unconditional release but it allows the police to continue their investigations."
Born in the UK, Ms Thornycroft is now a Zimbabwean citizen.
She is the first journalist to be detained since a media bill was signed into law days after Mr Mugabe's controversial re-election earlier this month.
Independent journalists have complained of a campaign of state harassment for a number of years.
The printing press of the only privately-owned daily newspaper, The Daily News, was bombed in 2001 and no arrests have been made.
In 1998, two journalists were illegally detained by the military and tortured for several days.
Now widowed, Mrs Thornycroft has a son, daughter and grand-daughter.
She has worked for the Daily Telegraph since last June and also works for South Africa's Mail and Guardian newspaper.