The nearly 15-hour siege on the Garissa University College campus was the worst attack on Kenyan soil since the U.S. embassy was bombed in 1998.
The Pentagon said Tahliil Abdishakur's "death will significantly impact al Shabaab's ability to conduct attacks."
A Somali Islamist group leader, who has a $3 million bounty, has surrendered, according to a Somali government source and local media.
A spokesman of the militant group said it "targeted the enemies at a time they were celebrating Christmas."
Nearly 350,000 fled war and conflict by sea in 2014, the UN Human Rights Agency said Wednesday.
At least 36 non-Muslim workers were killed, including two being beheaded
Though operations continue in Pakistan and Yemen, Somalia has third consecutive drone-free month.
Gunmen ordered non-Muslims off a bus Saturday and shot 28 dead. Muslim passengers were spared.