Ahead of MSPs returning to the Scottish Parliament on Tuesday, Mr Leonard said the team will "renew the energy" across the party ahead of the 2021 elections.
Among the new additions are former transport minister Sarah Boyack, who has taken over Kezia Dugdale's Lothians region seat and has been appointed as local government spokeswoman.
Joe Cullinane, the leader of North Ayrshire Council, has meanwhile been appointed to head up campaigns and party engagement for Scottish Labour.
Mr Leonard said: "We begin this new parliamentary term at one of the most important times in our political history.
"I am confident that our new shadow Cabinet team have the expertise, experience and enthusiasm to both hold the SNP Government to account and to put forward the distinctive Scottish Labour alternative.
"The new team will renew the energy across our party ahead of the 2021 Holyrood elections."
In other changes, James Kelly has been appointed as justice spokesman to replace Daniel Johnson, who stepped down from the frontbench in May.
Rhoda Grant succeeds Mr Kelly to become finance spokeswoman.
Alex Rowley has been handed the brief for Brexit and the constitution, while Elaine Smith takes up the role as business manager.
Iain Gray will be the education, skills and science spokesman for the party in the new parliamentary term, with Monica Lennon retaining her position as the party's health and sport spokeswoman.
Claire Baker continues as culture, tourism and external affairs spokeswoman for the party, while Colin Smyth retains his position as rural economy, transport and connectivity spokesman.
David Stewart replaces Elaine Smith as spokesman for the eradication of poverty and social inequality, while Claudia Beamish is spokeswoman for environment, climate change and land reform.
Pauline McNeill will serve in the frontbench as spokeswoman for communities and equalities.
The Scottish Labour leader added: "We know that we need to make a strong case based on Labour values with voters and work closely with our dedicated members across the country.
"That is why the Leader of North Ayrshire Council, Joe Cullinane, has been appointed to lead on campaigns and party engagement.
"He is a seasoned campaigner working hard in his local community and the experience he brings will be a great asset to our team.
"I am also delighted to welcome Sarah Boyack back to the Scottish Parliament and to our shadow Cabinet in the local government role.
"Sarah will be a strong addition to our team."