The African Union Mission in Somalia termed Mukhtar’s decision courageous and urged other fighters in the terror group to surrender to authorities.
“Amisom hopes that other sons and daughters of Somalia who have been misled into terrorist acts will [emulate Mukhtar],” the mission said via Twitter.
“[They should] lay down their weapons and join other Somalis in rebuilding their country,” it added, noting the government had offered amnesty.
The insurgency in Somalia wants to topple the government and impose its own strict interpretation of Islam.
Al Shabaab ruled most of south-central Somalia until 2011 when it was driven out of Mogadishu by African Union troops.
Despite the loss of territory, the terror group still carries out major gun and bomb attacks against nations that have contributed troops to Amisom.
On March 2, Kenya said its forces had killed 57 militants in a battle in southern Somalia but the group denied any of its fighters had died in the clash.
Kenya Defence Forces spokesman Colonel Joseph Owuoth said the ambush took place 31 kilometres northwest of Afmadow at 8.45am.
He said the troops used artillery and helicopter gunships against the militants.
More on this: KDF kills 57 al Shabaab militants in Somalia
In January, al Shabaab fighters attacked a Kenyan military base in the southern Somali town of Kulbiyow near the border.
KDF said nine soldiers died, while al Shabaab said it killed at least 66.
Somalia has been torn apart by civil war since 1991 and now a drought threatens to tip the Horn of Africa nation into famine.
Last month parliamentarians elected a new president who vowed to stamp out al Shabaab.