The Dumbarton MSP defeated Glasgow councillor Matt Kerr by 10,311 votes to 7,528 votes in the result announced today
Ms Baillie said the party has a “huge challenge” ahead with the Scottish Parliament elections looming next year, but that the immediate focus would be on the Coronavirus pandemic.
"When this election started, nobody expected the country to be in the grip of a global pandemic,” Ms Baillie said.
“Our collective focus will remain on the challenge that COVID-19 poses to the very fabric of our society.
"Our heartfelt thanks goes to the workers on the frontline – from doctors and nurses to social care staff, police officers and food retail workers. They are helping to care for us and keep us safe at a time of crisis and we should never forget their bravery and service.
"We will get through this together.
"It will be some time before politics returns to normal, but the next Scottish Parliament elections are now just over one year away.
"The Scottish Labour Party has a huge challenge ahead of us. Working together with Richard Leonard, our new UK leader and the entire Labour movement, I am ready for that challenge.”
Ms Baillie had called for greater autonomy from London control for the Scottish party during her campaign and a hard line against another independence referendum. She succeeds Lesley Laird who lost her seat as Kirkcaldy and Cowdenbeath MP in last year’s Westminster election.
Labour leader Richard Leonard had previously sacked Ms Baillie from his front bench, but insisted they can bring the party together.
”We will unite the Party and work closely together to get the party in shape to fight the 2021 Scottish Parliament elections,” he said.
“Jackie will bring a renewed energy and determination to win so I am extremely pleased that she is joining the leadership team at this critical time.”