Kathleen Marshall, a visiting professor at Glasgow University, was selected for the pound;72,000 post by a panel of MSPs chaired by George Reid, the Parliament's presiding officer, and which included Robert Brown, convener of its education committee.
Mr Brown said the "landmark" decision to make such an appointment required the commissioner "actively to encourage the involvement of children and young people in their work".
Candidates had to hold discussions with a group of 20 young people who then prepared a report for the MSPs on what they thought of the applicants.
Professor Marshall, aged 50, made her name in the early 1990s as director of the Scottish Child Law Centre. She pledged that her first priority will be "to set up ways in which children and young people can get their issues on the public agenda".
She has already stirred up the political community with a strong attack on locking up children at the Dungavel asylum detention centre, a policy which resulted from Westminster's control over refugee matters and has led to considerable embarrassment for the Scottish Executive.
The remit of the commissioner is to "promote and safeguard" the rights of children and young people up to the age of 18, and up to 21 in certain defined circumstances.