Tareena Shakil was found guilty at Birmingham Crown Court on Friday of joining IS and encouraging terrorism on social media.
The 26-year-old boarded a plane to Turkey in October 2014 with her one-year-old boy, crossed the border into Syria and spent three months there, West Midlands police said in a statement.
She denied joining IS but detectives said Shakil had become a member of the extremist group and was set to become a jihadi bride.
A photograph uncovered by police showed her posing in Syria underneath an IS flag. She left the country in January 2015, although it is not known why, the statement added.
She was arrested by counter-terrorism officers when she returned to Britain on Feb. 18 after landing at Heathrow Airport. The child was taken into care.
“Tareena Shakil had self-radicalised by viewing extremist material on the internet, before leaving the UK,” said Assistant Chief Constable Marcus Beale in the statement.
“Our assessment is that she was not naïve. She had absolutely clear intentions when she left the UK, sending tweets encouraging the public to commit acts of terrorism here and then taking her young child to join Daesh (IS) in Syria.
“Photographs seized from her phone showed Ms Shakil posing with a firearm and wearing a Daesh balaclava. Another showed a rucksack with a Daesh logo and person holding a handgun. These were taken while she was in Syria.”
Security services estimate some 600 Britons have joined the ranks of Islamic State and other militant groups in Syria and Iraq, many of them crossing via Turkey. About half are believed to have returned to Britain.
Shakil will be sentenced on Monday.